This is an informal commemorative day to celebrate the Star Wars media franchise. The date comes from the saying in the film “May the Force be with You”, which is turned into “May the Fourth be with You.”
This has now turned into a day where new series or tv shows are released to do with Star Wars, as well as fans watching the films.
Some recognise the day after as Revenge of the Fifth and the 6th May as Revenge of the Sixth, both a play on the third film “Revenge of the Sith”. These days though celebrate the Sith and other villanious characters.