Star Wars Day, May 4, celebrates George Lucas’s Star Wars. It is observed by fans of the media franchise. Observance of the commemorative day spread quickly through the media and grassroots organizations.
The date was chosen for the pun of the catchphrase “May the Force be with you.” Even though the holiday was not actually created or declared by Lucasfilm, many Star Wars fans across the world have chosen to celebrate the holiday. It has since been embraced by Lucasfilm as an annual celebration of Star Wars.
Taken from Wikipedia.
Actually, the first reference was used May 4, 1979, the day Margaret Thatcher took office as the first female Prime Minister of the United Kingdom.
Her political party, the Conservatives, placed a congratulatory advertisement in The London Evening News saying “May the Fourth Be with You, Maggie. Congratulations.”