What are the reasons for the European Court of Justice to declare a legal act of the EU invalid?

Question

The Article 267 of the Treaty on the Functioning of the European Union (TFEU) provides:

The Court of Justice of the European Union shall have jurisdiction to give preliminary rulings concerning:
(a) the interpretation of the Treaties;
(b) the validity and interpretation of acts of the institutions, bodies, offices or agencies of the Union;
[...]

What are the reasons to declare a legal act of the EU (such as regulation or directive) invalid?

The Article 263 of the TFEU provides:

The Court of Justice of the European Union shall review the legality of legislative acts, of acts of the Council, of the Commission and of the European Central Bank, other than recommendations and opinions, and of acts of the European Parliament and of the European Council intended to produce legal effects vis-à-vis third parties. It shall also review the legality of acts of bodies, offices or agencies of the Union intended to produce legal effects vis-à-vis third parties.

What are the differences between the validity (under the Article 267) and the legality (under the Article 263)?

Answer 1

Les Verts v. Parliament (Case 294/83) paragraph 23 seems to have the most concise answer to your questions despite it being when the EU was still the EEC:

It must first be emphasized in this regard that the European Economic Community is a Community based on the rule of law, inasmuch as neither its Member States nor its institutions can avoid a review of the question whether the measures adopted by them are in conformity with the basic constitutional charter, the Treaty. In particular, in Articles 173 [modern TFEU 263] and 184 [TFEU 277], on the one hand, and in Article 177 [TFEU 267], on the other, the Treaty established a complete system of legal remedies and procedures designed to permit the Court of Justice to review the legality of measures adopted by the institutions. [...]

For modern restatements see for example C‑583/11 paragraph 92 or C‑72/15 paragraph 66.

Ultimately, all EU acts must be consistent with the "basic constitutional charter," these days that's the Treaty on European Union (TEU), the Treaty on the Functioning of the European Union (TFEU) and by reference, the Charter of Fundamental Rights of the European Union.

While Regulations and Directives could only be declared invalid on the basis that they violate the treaties, secondary legislation or actions could additionally be declared invalid under whatever Regulation or Directive they were adopted.

For a Directive invalidity example, see Digital Rights Ireland and Seitlinger and Others (Joined Cases C‑293/12 and C‑594/12) where the Data Retention Directive was declared invalid¹, being contrary to Charter Articles 7, 8 and 52(1).

As to "validity" versus "legality", there appears to be no real difference, as the court is very much stating the equivalence of the two Articles in this respect.

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1. This appears to be the only time a Directive has been declared invalid. Or at least the only one Google searches readily turn up, and you would think the articles discussing this would have mentioned any previous instance. It triggered much speculation as to what the status of the national transpositions were, one I'm not sure was ever fully resolved.