Short answers, pulled from the story.
Star Wars Day is observed on the 4th of May because the date sounds like "May the Force be with you," the franchise's famous phrase, when spoken aloud as "May the Fourth be with you." The pun drives the entire holiday, which was not created by Lucasfilm but grew through fan and grassroots observance.
The first recorded use appeared on the 3rd of May, 1979, in a half-page Conservative Party advertisement on page 13 of the London Evening News, congratulating Margaret Thatcher on her election as Prime Minister with the line "Dear Maggie, May the Fourth Be with You. Your Party Workers."
The first organized Star Wars Day celebration took place in 2011 at the Toronto Underground Cinema in Toronto, Ontario, Canada. It was produced by Sean Ward and Alice Quinn and featured a trivia game show, a costume contest with celebrity judges, and a screening of fan-made tribute films.
Disney began officially observing Star Wars Day in 2013, one year after purchasing Lucasfilm in late 2012. The company has since held annual Star Wars events and festivities at Disneyland and Walt Disney World, and has released films and series on Disney+ each the 4th of May.
Revenge of the Fifth is an informal observance on the 5th of May that focuses on Sith Lords and villainous Star Wars characters, playing on the title Star Wars: Episode III - Revenge of the Sith. Some fans prefer the 6th of May instead, calling it "Revenge of the Sixth" or "Return of the 6th."
Yes. The Los Angeles City Council declared the 25th of May, 2007, as Star Wars Day to mark the 30th anniversary of the original film's release on the 25th of May, 1977. May 25 is also observed by some as Geek Pride Day, with connections to The Hitchhiker's Guide to the Galaxy and Discworld.