I hate to be the devil on your shoulder but one night of celebration isn't going to push you over the edge too terribly much. Just stay away from mixed drinks and enjoy some wine or a martini. And STAY AWAY FROM BEER BLOAT!
I'm sorry but diet or no diet, I cannot imagine letting my 21st birthday go by without celebration. In a country where you can die for your country before you can have a drink, it's a celebration that needs to be had. Just be safe and don't drink and drive.
Enjoy your birthday and then just be extra good the next few days. Happy Birthday, btw!