In the US, the most popular birthday date is September 16. This date is followed in popularity by September 9 and September 23. The least popular birthday date is February 29, occurring only every four years. The second least popular date is January 1, followed by December 24 and 25.
The large concentration of births in September, followed closely by many births in August, is thought to be due to the holiday season, occurring roughly nine months earlier. This period is popular for couples to conceive. January is one of the least popular months for births, and several dates in December are among those with the lowest birthrate.