was it the last 5 minutes? i saw it some time ago and i can't remember it well but i distinctly remember that Mai was having her graduation in Kazahana...
either way, its just a special. nothing earth shattering happens. basically, a slice of life with the Kanon charaters all leading up to the final scene in the original Kanon series
actually, now that i remember it, you get this special DVD in japan if you BUY enough Kanon merchandise. something like 6000 yen worth or something like that.