The most common birthday in the United States is September 16, based on National Vital Statistics Reports of people born between 1973 and 1999. The least common birthdays are December 25 and February 29, according to information published by Amitabh Chandra of Harvard University.
Overall, the months with the least number of births were the winter months of November, December and January. The summer months of July, August and September had the highest number of births. For example, the top 10 most common birthdays all occur in September. However, the difference in the distribution of births between the highest and lowest months is very small.