The Birthday Problem paradox suggests that if you put 23 people into a room, there's a 50 percent chance that two of them share the same birthday (and that probability jumps to 99 percent chance with just 60 people). The math is based on the assumption that every date out of the 365 days a year is equally likely for a birthday, but apparently that theory is way off.
In 2006, Amitabh Chandra of Harvard University analyzed the birth dates of every baby born in the United States between January 1, 1973, and December 31, 1999, and published the findings in a chart on the New York Times. The most popular birth months were July, August and September, with all 10 of the top 10 birthdays taking place during the latter month.
The least likely birthdays fell around holidays. Christmas, December 25th, is the least popular birthday (when you leave leap day, February 29th, out of the equation) and New Year's Day, January 1st, follows right behind. This is surely due to mothers postponing inductions and scheduled deliveries; the days leading up to Christmas rank low for births, as well, but births spike between December 28 and 30, presumably to deliver those babies that were kept cooking over the holiday.
Wondering how popular your birthday is? Andy Kriebel of Viz Wiz took the New York Times chart and turned it into an interactive heat map. To find your birthday, look for your birth month across the top row, then follow the column down to match your birth date on the left and discover where your birthday ranks among the 366 days of the year.
How Popular is Your Date of Birth?