I don't visit the official starwars website very often, so by chance I followed a link or two that resulted in stumbling across a "talk like yoda day" thread on the starwars.com's forums. Way back in 2005 on August 13th, one of its registered users suggested that a Talk Like Yoda Day was in order. He also promoted the date for it as May 21st, considering this was the day the world first saw, and, more importantly, heard, our little green friend.
Of course, I then registered on the starwars forums and mentioned I had this little yodaspeak site that any Yoda-talkers may find handy.
As the thread continued for several pages, the date being discussed became May 22nd. A few reasons can be told by lumpy, but the main one I think was that May 21st 2006 (when it was first aimed at) was a Sunday and hence would not give everyone a chance to use Yoda-speak at work or at school.
I've left it as May 22nd on here because I made the link to their thread first. Since and before then I've come across other "Talk Like Yoda" sites, such as the rather unavoidable talklikeyoda.com, a site which started in September 2006, who mention May 21st.
It's not really important enough to worry about, so you just go ahead and do it on whichever day suits you... or do it on both days! And why not host a "Talk Like Yoda Weekend"? The possibilities are endless. This site is just for how to do it, not when. :)