I turn 47 today, so here are some facts about birthdays:

1. September 16th is the most common birthday in America according to data collected by a Harvard economist of all births in the U.S. from 1973 to 1999. Today, April 6th is the 286th most common birthday.
2. August is the most common month for birthdays in the U.S.  The second most common is September.  So, most conceptions occur in the winter. Why?  One study postulated a number of theories including: “deterioration of sperm quality during summer, seasonal differences in anterior pituitary-ovarian function caused by changes in the daylight length, and variation in quality of the ovum or endometrial receptivity. Or, it could be the increased sexual activity that comes with end-of-the-year festivities.”
3. The least common birthday in February 29th, for obvious reasons, followed by December 25th.
4. The song Happy Birthday to You was actually a song written in 1893 for kindergartners called “Good Morning to You”. At some point the lyrics were changed to happy birthday.  According to the Guiness Book of World Records it is the most recognized song in the English language.
5. Celebration of birthdays have pagan origins dating back before Christianity. It was  believed evil spirits visited people on their birthdays. To protect the  birthday person from evil, people used to surround him and make merry. They would make a lot of noise to scare away the evil spirits.
6. Why do we put candles on our cakes and then blow them out? There are competing theories.  There is some evidence that tie the tradition to ancient Greece and worship of the god Artemis, other evidence that it is pagan spiritual ritual and other evidence that the practice began in 18th century Germany.  So – we don’t really know the origins or why we do it.  We just all do it for some reason.  Some health officials recommend against blowing out candles on birthday cakes because multiple studies have found that blowing out candles spreads germs and bacteria all over the cake.
7. If there are 23 people in a room there is a 50-50 chance that two of them will share a birthday. This is known as the birthday paradox, because it appears there are only 22 possible pairs and therefore a low chance of a match. In fact, because every guest could match with every other guest, there are 253 possible combinations (1+2+3+4… up to 22) and the probability of finding a successful one is 50.048 per cent. The probability rises to 94% for a room of 45 people.

From the study referenced in 1, above, here’s a link to a table which shows the commonality rank of each day.  You can look up your birthday rank: http://www.nytimes.com/2006/12/19/business/20leonhardt-table.html?_r=1