From the official Star Wars site:
One of the earliest known records of “May the 4th” used in popular culture is in 1979, as described here by author Alan Arnold while he was chronicling the making of The Empire Strikes Back for Lucasfilm: “Margaret Thatcher has won the election and become Britain’s first woman prime minister. To celebrate their victory her party took a half page of advertising space in the London Evening News. This message, referring to the day of victory, was ‘May the Fourth Be With You, Maggie. Congratulations,’ further proof of the extent to which Star Wars has influenced us all.” May the 4th kicks off a season of celebration, particularly since the month of May has always been important to Star Wars fans. The six live-action movies of the Star Wars saga debuted in May, starting with the original Star Wars on May 25, 1977.