It's May 4th, which in 2011 became a Star Wars celebration Day - a play on the first-religious-then-biological "may the force be with you" from the films.
"May the fourth be with you" had nothing to do with "Star Wars" initially, though obviously the hit phenomenon inspired it. It was used as congratulations to new UK Prime Minister Margaret Thatcher, who took office on that day in 1979. Then it showed up in a 1988 episode of "Count Duckula" (
The Vampire Strikes Back) spoofing the movies. Toronto had the first real celebration of it in 2011 and it took off from there.