For those of you who don't know, May 4th is considered Star Wars Day.
May 4 is called Star Wars Day because of a pun or play on words based 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.
This all came about because some over zealous fans wanted to start a Jedi Church with May 4th being it's main holiday, Star Wars Day. As of March 4th, 2009, there are no religion-supporting organization that promotes March 4th.
Instead, fans take it upon themselves to celebrate this day with as much enthusiasm as possible. One thing that is popular is called a fan edit. Fan edits are where people film themselves in front of a blue or green screen, take clips from any of the six Star Wars movie, and composite them together to get their take on the Star Wars universe.
Despite not having an official or religious connection to the Star Wars franchise, there WAS an official Star Wars Day at one point in time. This was a one day event where Los Angeles City Council in California declared May 25, 2007 Star Wars Day due to being the 20 year anniversary of the film's initial release in 1977.
So, make today memorable by making your own fan edit, watching one or all of the Star Wars movies, or just remembering what it was like watching Star Wars for the very first time.