THE date for Star Wars fans is, of course, May 25, 1977, when the very first film was released way back when; but I see via The Dude that there's actually an earlier date: April 17, 1973, which is when Lord George, Duke of Lucas, sat down with legal pad and No. 2 pencil in hand to start cobbling together his first draft of The Star Wars.
For we obsessives who have delved into those early drafts (The Jedi Bendu Script Site is an invaluable resource), it's fascinating to see which elements of Lucas's first writings made it into the first films, and then to further see everything come full circle as some of those very early notions, set aside for the Original Trilogy, eventually turned up in the Prequel Trilogy (the planet Utapau, Darth Maul's attack on Qui Gon in the Tatooine desert, and so on). It all started coming together way back in 1973.
(By the way, the image here is taken from a Flickr gallery of Ralph McQuarrie's amazing conceptual art for the Star Wars films. His art was instrumental in selling the concept to the studios, and is well-worth checking out. Be sure to click through to the large version of this early poster concept, just to read the dummy credits at the bottom. Try to imagine Star Wars with songs sung by Andy Williams!)