For some bizarre reason, it was thought in the time of Pius XII that making Mayday, which had been adopted by communists and socialists as their holiday, the feast of St Joseph the Worker, communism and socialism would be utterly defeated, or neutralised, or something, or at least it would give Italian men an excuse to have the day off on 1 May and walk up and down a bit. This is the sort of thing that happens when you take the Papal States off somebody and don't define their new job properly.
The problem is that 1 May already is a feast, and an important one at that. It is the feast of SS Philip and James, two of the twelve apostles. So they have to be moved (they can't be ignored). They take over 11 May. That is the feast of Pope St Alexander I in Rome (and St Francis of Jerome elsewhere, but elsewhere is expendable) so St Alexander has to be moved to 3 May, where he in turn displaces the Finding of the Holy Cross which can be merged with 14 September with the Feast of the Exaltation of the Cross.
We talk about wreckovation of churches as a "fruit" of the Spirit of Vatican II: wreckovation of the calendar began a long time before.