### Instruction: What is the least common birthday in the United States?
### Input: 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 (other than February 29, because of leap years). In 2011, October 5 and 6 were reported as the most frequently occurring birthdays.
### Response: The least common birthday in the United States between 1973 and 1999 was February 29 because of leap years.