In our house we do half-birthdays. Not the adults, not unless time suddenly starts reversing, but for my son and daughter. It’s not a tradition carried over from my childhood or my husband’s but rather it started when my son turned 2 1/2.

I was home on maternity leave with my new daughter and wondering (lamenting, if I am being truthful) at how quickly time was moving forward. Surely it was only a few weeks ago that Padraig was this tiny! It suddenly occurred to my addled, sleep-deprived mind that he was actually 2 1/2 that very day. Part of me was immediately shamed; when he was my one and only I mentally marked every week, but suddenly having to share the spotlight with his sister meant Mom was slipping on a few things. I decided that I would make him half a cake to celebrate after dinner. He loved banana muffins at the time, so I made the batter and cooked it in a 9-inch pan. I slapped frosting on it, cut it in half, stacked it and viola! A tradition began – half a cake on your half-birthday.

http://www.tastebook.com/blog/you-say-its-your-half-birthday/