Dictionary.com defines Birthday as:
3) a day marking or commemorating the origin, founding, or beginning of something.
Urban Dictionary defines Birthday as:
1) The one day of the year that you should be able to do whatever you want, but ends up being the day that you do what everyone else thinks you want.
Ricky: "I'm good with just staying home and watching Football..."
Family/Friends: "No! It's your Birthday! Come on, let's go out for dinner/drinks/socializing!"
The Heavy Jack 3 define Birthday as: