May the Fourth is celebrated as Star Wars Day, as a play on the words May the Force [Be With You.] Even though the holiday was not created or declared by Lucasfilm, many Star Wars fans across the world have chosen to celebrate the holiday.
Some ardent Star Wars fans also celebrate either tomorrow, May the Fifth or the day after tomorrow, May the Sixth as a play on Revenge of the Sith when they celebrate the Sith Lords and other villainous characters from Star Wars.
Ironically, the first use of the term was not in connection with Star Wars at all. It was an advertisement from the Conservative Party congratulating Margaret Thatcher on winning the PM seat in 1979.