Mine was on a FREAKING Sunday, when all the cool bars were closed. I went in the liquor store Sat night, at 11:58(two minutes before they closed), and didn't get to the checkout until 12:00(figuring that WHEN they carded me, they couldn't do anything, since it was now technically my Bday. Those bastards didn't even card me!!! I was soooo pissed, I mean, could I have been going in there for an additional year or two, if I'd had the balls to try it?
My BEST 21 Bday memory is taking my best friend from High School out for his 21st. He got totally wasted, and was running for the bathroom, and threw up, ALL OVER THIS BIG GUY IN A COWBOY HAT!

All down the cowboy's pant leg! I apologized profusely for him, and told him that it was his 21st, and ODDLY, the cowboy was FINE with it! I'd have bet that we were about to be castrated.