What is the least common birthday in the United States?

Context: In the United States between 1973 and 1999, September 16 is the most common birthday in the United States and December 25 the least common birthday. In 2011, October 5 and 6 were reported as the most frequently occurring birthdays.

Answer: The least common birthday in the United States between 1973 and 1999 was February 29 because of leap years.