18) By its first question, the referring court asks, in essence, whether Article 7 of Directive 2003/88 and Article 31(2) of the Charter must be interpreted as precluding national legislation, such as that at issue in the main proceedings, under which, in the event that the worker did not ask to be able to exercise his right to paid annual leave during the reference period concerned, that worker loses, at the end of that period, the days of paid annual leave acquired under those provisions in that period, and, accordingly, his entitlement to payment of an allowance in lieu of annual leave not taken where the employment relationship is terminated.

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20) Moreover, the right to paid annual leave, as a principle of EU social law, is not only particularly important, but is also expressly laid down in Article 31(2) of the Charter, which Article 6(1) TEU recognises as having the same legal value as the Treaties (judgment of 30 June 2016, Sobczyszyn, C‑178/15, EU:C:2016:502, paragraph 20 and the case-law cited).

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49) As regards, secondly, Article 31(2) of the Charter, it should be recalled that it is settled case-law of the Court that the fundamental rights guaranteed in the legal order of the European Union are applicable in all situations governed by EU law (see, inter alia, judgment of 15 January 2014, Association de médiation sociale, C‑176/12, EU:C:2014:2, paragraph 42 and the case-law cited).

50) Since the legislation at issue in the main proceedings is an implementation of Directive 2003/88, Article 31(2) of the Charter is intended to apply to the case in the main proceedings (see, by analogy, judgment of 15 January 2014, Association de médiation sociale, C‑176/12, EU:C:2014:2, paragraph 43).

51) In that regard, it follows, first, from the wording of Article 31(2) of the Charter that that provision enshrines the ‘right’ of all workers to an ‘annual period of paid leave’.

52) Next, according to the explanations relating to Article 31 of the Charter, which, in accordance with the third subparagraph of Article 6(1) TEU and Article 52(7) of the Charter, must be taken into consideration for the interpretation of the Charter, Article 31(2) of the Charter is based on Directive 93/104 and Article 2 of the European Social Charter, signed in Turin on 18 October 1961 and revised in Strasbourg on 3 May 1996, and on point 8 of the Community Charter of the Fundamental Social Rights of Workers, adopted at the meeting of the European Council in Strasbourg on 9 December 1989 (judgment of 19 September 2013, Review of Commission v Strack, C‑579/12 RX-II, EU:C:2013:570, paragraph 27).

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54) In that context, it should, finally, be recalled that limitations may be imposed on the fundamental right to annual paid leave affirmed in Article 31(2) of the Charter only in compliance with the strict conditions laid down in Article 52(1) thereof and, in particular, the essential content of that right. Thus, Member States may not derogate from the principle flowing from Article 7 of Directive 2003/88 read in the light of Article 31(2) of the Charter, that the right to paid annual leave acquired cannot be lost at the end of the leave year and/or of a carry-over period fixed by national law, when the worker has been unable to take his leave (see, to that effect, judgment of 29 November 2017, King, C‑214/16, EU:C:2017:914, paragraph 56).

55) It follows from those considerations that both Article 7 of Directive 2003/88 and, as regards situations falling within the scope of the Charter, Article 31(2) thereof, must be interpreted as meaning that they preclude national legislation pursuant to which the fact that a worker has not requested to exercise his right to paid annual leave acquired under those provisions during the reference period automatically entails, without prior verification that the worker has actually been given the opportunity to exercise that right, the consequence that the worker loses the benefit of that right and, accordingly, his entitlement to an allowance in lieu of paid annual leave not taken in the event of the termination of the employment relationship.

56) On the other hand, where the worker has refrained from taking his paid annual leave deliberately and in full knowledge of the ensuing consequences, after having been given the opportunity actually to exercise his right thereto, Article 7(1) and (2) of Directive 2003/88 and Article 31(2) of the Charter do not preclude the loss of that right or, in the event of the termination of the employment relationship, the corresponding absence of an allowance in lieu of paid annual leave not taken, without the employer being required to force that worker to actually exercise that right.

57) It is for the referring court to ascertain whether the national legislation at issue in the main proceedings may be interpreted in accordance with Article 7(1) and (2) of Directive 2003/88 and Article 31(2) of the Charter.

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61) In the light of the foregoing, the answer to the first question is that Article 7 of Directive 2003/88 and Article 31(2) of the Charter must be interpreted as precluding national legislation, such as that at issue in the main proceedings, under which, in the event that the worker did not ask to exercise his right to paid annual leave during the reference period concerned, that worker loses, at the end of that period — automatically and without prior verification of whether the employer had in fact enabled him to exercise that right, in particular through the provision of sufficient information — the days of paid annual leave acquired under those provisions in respect of that period, and, accordingly, his right to an allowance in lieu of paid annual leave not taken in the event that the employment relationship is terminated. It is, in that regard, for the referring court to determine, taking into consideration the whole body of domestic law and applying the interpretative methods recognised by it, whether it can arrive at an interpretation of that right capable of ensuring the full effectiveness of EU law.

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62) By its second question, the referring court asks, in essence, whether, in the event that it is impossible to interpret national legislation such as that at issue in the main proceedings in a manner consistent with Article 7 of Directive 2003/88 and Article 31(2) of the Charter, those provisions of EU law must be interpreted as meaning that, in the context of a dispute between the worker and his former employer who is a private individual, they result in the national legislation having to be disapplied by the national court, and the worker having to be granted by the employer an allowance in lieu of the annual leave acquired under those provisions and not taken at the time that the employment relationship was terminated.

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69) As regards, secondly, Article 31(2) of the Charter, a provision for which it was established, in paragraphs 49 to 55 of the present judgment, that it is intended to apply to situations such as those in the main proceedings and must be interpreted as meaning that it precludes legislation such as that at issue in the main proceedings, it should be recalled at the outset that the right to paid annual leave constitutes an essential principle of EU social law.

70) That principle is itself mainly derived both from instruments drawn up by the Member States at EU level, such as the Community Charter of the Fundamental Social Rights of Workers, which is moreover mentioned in Article 151 TFEU, and from international instruments on which the Member States have cooperated or to which they are party. Among them is the European Social Charter, to which all Member States are parties in so far as they are party to it in its original version, its revised version or in both versions, also referred to in Article 151 TFEU. Mention should also be made of Convention No 132 of the International Labour Organisation of 24 June 1970 concerning Annual Holidays with Pay (revised) which, as the Court noted in paragraphs 37 and 38 of the judgment of 20 January 2009, Schultz-Hoff and Others (C‑350/06 and C‑520/06, EU:C:2009:18), sets out principles of that organisation which recital 6 of Directive 2003/88 states must be taken account of.

71) In that regard, the fourth recital of Directive 93/104 states, in particular, that paragraph 8 of the Community Charter of the Fundamental Social Rights of Workers provides that every worker in the Union has a right, inter alia, to paid annual leave, the duration of which must be progressively harmonised in accordance with national practices (see, to that effect, judgment of 26 June 2001, BECTU, C‑173/99, EU:C:2001:356, paragraph 39).

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73) By providing, in mandatory terms, that ‘every worker’ has ‘the right’‘to an annual period of paid leave’ without referring in particular in that regard — like, for example, Article 27 of the Charter which led to the judgment of 15 January 2014, Association de médiation sociale (C‑176/12, EU:C:2014:2) — to the ‘cases’ and ‘conditions provided for by Union law and national laws and practices’, Article 31(2) of the Charter, reflects the essential principle of EU social law from which there may be derogations only in compliance with the strict conditions laid down in Article 52(1) of the Charter and, in particular, the fundamental right to paid annual leave.

74) The right to a period of paid annual leave, affirmed for every worker by Article 31(2) of the Charter, is thus, as regards its very existence, both mandatory and unconditional in nature, the unconditional nature not needing to be given concrete expression by the provisions of EU or national law, which are only required to specify the exact duration of annual leave and, where appropriate, certain conditions for the exercise of that right. It follows that that provision is sufficient in itself to confer on workers a right that they may actually rely on in disputes between them and their employer in a field covered by EU law and therefore falling within the scope of the Charter (see, by analogy, judgment of 17 April 2018, Egenberger, C‑414/16, EU:C:2018:257, paragraph 76).

75) Article 31(2) of the Charter therefore entails, in particular, as regards the situations falling within the scope thereof, that the national court must disapply national legislation negating the principle, recalled in paragraph 54 of this judgment, that a worker cannot be deprived of an acquired right to paid annual leave at the end of the leave year and/or of a carry-over period fixed by national law when the worker has been unable to take his leave, or correspondingly, of the entitlement to the allowance in lieu thereof upon termination of the employment relationship, as a right which is consubstantial with the right to ‘paid’ annual leave. Under that provision, nor may employers rely on that national legislation in order to avoid payment of the allowance in lieu which they are required to pay pursuant to the fundamental right guaranteed by that provision.

76) With respect to the effect of Article 31(2) of the Charter on an employer who is a private individual, it should be noted that, although Article 51(1) of the Charter states that the provisions thereof are addressed to the institutions, bodies, offices and agencies of the European Union with due regard for the principle of subsidiarity and to the Member States only when they are implementing EU law, Article 51(1) does not, however, address the question whether those individuals may, where appropriate, be directly required to comply with certain provisions of the Charter and cannot, accordingly, be interpreted as meaning that it would systematically preclude such a possibility.

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78) Next, the Court has, in particular, already held that the prohibition laid down in Article 21(1) of the Charter is sufficient in itself to confer on individuals a right which they may rely on as such in a dispute with another individual (judgment of 17 April 2018, Egenberger, C‑414/16, EU:C:2018:257, paragraph 76), without, therefore, Article 51(1) of the Charter precluding it.

79) Finally, as regards, more specifically, Article 31(2) of the Charter, it must be noted that the right of every worker to paid annual leave entails, by its very nature, a corresponding obligation on the employer, which is to grant such periods of paid leave or an allowance in lieu of paid annual leave not taken upon termination of the employment relationship.

80) In the event that it is impossible to interpret the national legislation at issue in the main proceedings in a manner consistent with Article 31(2) of the Charter, it will therefore be for the referring court, in a situation such as that at issue in the main proceedings, to ensure within its jurisdiction the judicial protection for individuals flowing from that provision and to guarantee the full effectiveness thereof by disapplying if need be that national legislation (see, by analogy, judgment of 17 April 2018, Egenberger, C‑414/16, EU:C:2018:257, paragraph 76).

81) In the light of all the foregoing considerations, the answer to the second question is that, in the event that it is impossible to interpret national legislation such as that at issue in the main proceedings in a manner consistent with Article 7 of Directive 2003/88 and Article 31(2) of the Charter, it follows from the latter provision that a national court hearing a dispute between a worker and his former employer who is a private individual must disapply the national legislation and ensure that, should the employer not be able to show that it has exercised all due diligence in enabling the worker actually to take the paid annual leave to which he is entitled under EU law, the worker cannot be deprived of his acquired rights to that paid annual leave or, correspondingly, and in the event of the termination of the employment relationship, to the allowance in lieu of leave not taken which must be paid, in that case, directly by the employer concerned.

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82) Since these proceedings are, for the parties to the main proceedings, a step in the action pending before the national court, the decision on costs is a matter for that court. Costs incurred in submitting observations to the Court, other than the costs of those parties, are not recoverable.

On those grounds, the Court (Grand Chamber) hereby rules:

1. Article 7 of Directive 2003/88/EC of the European Parliament and of the Council of 4 November 2003 concerning certain aspects of the organisation of working time and of Article 31(2) of the Charter of Fundamental Rights of the European Union must be interpreted as precluding national legislation such as that at issue in the main proceedings, under which, in the event that the worker did not ask to exercise his right to paid annual leave during the reference period concerned, that worker loses, at the end of that period — automatically and without prior verification of whether the employer had in fact enabled him to exercise that right, in particular through the provision of sufficient information — the days of paid annual leave acquired under those provisions in respect of that period, and, accordingly, his right to an allowance in lieu of paid annual leave not taken in the event that the employment relationship is terminated. It is, in that regard, for the referring court to determine, taking into consideration the whole body of domestic law and applying the interpretative methods recognised by it, whether it can arrive at an interpretation of that right capable of ensuring the full effectiveness of EU law.

2. In the event that it is impossible to interpret national legislation such as that at issue in the main proceedings in a manner consistent with Article 7 of Directive 2003/88 and Article 31(2) of the Charter of Fundamental Rights, it follows from the latter provision that a national court hearing a dispute between a worker and his former employer who is a private individual must disapply the national legislation and ensure that, should the employer not be able to show that it has exercised all due diligence in enabling the worker actually to take the paid annual leave to which he is entitled under EU law, the worker cannot be deprived of his acquired rights to that paid annual leave or, correspondingly, and in the event of the termination of the employment relationship, to the allowance in lieu of leave not taken which must be paid, in that case, directly by the employer concerned.