May 4 is called Star Wars Day (also sometimes known as Luke Skywalker Day) because of a pun on the similarity between "May the 4th be with you" and "May The Force Be With You", a phrase often spoken in the Star Wars movies.
Zoe Whittaker came up with the idea of "May the 4th be with you" while they were shooting Star Wars Episode III in the UK Janurary 2005.
She told Rick McCallum and they all went over to tell George Lucas. This take was new to them and they liked it a lot. Zoe suggested that they release Episode III on this day. They both laughed and found it entertaining but everything was already set for the release on the 19th of May 2005 which could not be changed.
The phrase originally appeared in the Episode "The Vampire Strikes Back" of the British Cartoon "Count Duckula" in 1988.
May the 4th be with you!