Why Is "Star Wars Day" On May 4th?

Yesterday was "Star Wars Day" apparently, an occasion that passed me by.

When I saw a reference to it online just now, I wondered what significance May 4th has for Star Wars fans. A quick search revealed that apparently the date was chosen because it would result in a pun on "May the Force be with you" (i.e. "May the Fourth be with you").

Yeah.

This video made me laugh. I'm not sure whether that's despite, or because of, that pun.