I'm not sure how that tradition started, but it's definitely one I can get behind (you should pardon the expression). The most logical explanation, of course, is that it goes back to the moment right after birth, when the doctor holds the baby up and gives it a smack on the bum to help clear the lungs of fluid and get the child breathing--not to mention screaming it's lungs out. It's the only time when crying and screaming are looked upon as good things and signs of health.
Each year thereafter, in order to celebrate the blessed event, the birthday girl or boy receives a whack on the bottom for each year of life.
The thing that occurs to me, of course, is that this is maybe the one time that the Vanilla World and the Spanko World touch and coexist. Under normal circumstances, tell a girl that she should be spanked and she's liable to start backing away from you and reaching for her Mace. Tell her the same thing at her birthday party and you'd be surprised at how fast most will laugh and bend over. (Some mass consumption of alcohol first probably helps, but not always.)
When it comes to birthday spankings, the recipients seem to fall into four categories. First--sad but true--there's the girl who really, truly does not like it--
Next, there's the girl who doesn't really care for it but will put up with it to be a good sport--
Then there's the girl who just accepts it for the goofy good fun that it is--