December 21st.
I usually just hang out with my friends and family, do something silly, and eat good food. I've gone down to secluded parts of my local beach a couple of nights to watch the stars if it's not too cold. Don't know how that'll work out this year with the oil spill clogging up the shores, though.
For 2012, my friends are throwing me an end of the world celebratory barbecue. If the supposed last day on earth is going to be on my birthday, I might as well go out drunk and full of cake.
Also, happy birthday to all! =)