Answer: September

People in cooler climates love to talk about how the cold of winter leads to a baby boom nine months later. It turns out, they’re not far off. In fact, the month with the highest number of birthdays is September, a month conveniently and not coincidentally right around the span of the human gestation period after the double holiday of Christmas and New Year’s.

In fact, if you had to guess somebody’s birthday with single day precision, you’d be well served to guess September 9th, 19th, or 12th—the three most popular birth dates in the United States. Still, though, we wouldn’t recommend betting any large sums of money on your birthday guessing skills. While September is a clear mathematical winner for the prize of most birthdays, the margin it enjoys is small. The difference between the most popular birthday out of the year, September 9th, and the least popular one, December 25th, is just 5,727 births per year.