I realized that I knew several people with today, September 16th, as their birthday [Maria, Donna, Breyon, and Emily… at least]. That led me to wonder what the most common birthday was – well it turns out there are several answers. I have checked a number of studies and there seems to be three common answers: September 16th, September 30th and October 5th. That of course leads to the question of what is the average human gestation period? Answer: 280 days. Now, doing the math backwards from those three dates you get December 11th, December 25th, and December 31st. I guess two of those are no surprise.
By the way, besides February 29th, the least common birthday is December 25th.