What is the least common birthday?
The least common birthday in the modern Gregorian calendar is February 29. This is because of leap years, as February 29 occurs only a bit more than a quarter of the time in any given calendar year.

However excluding February 29, in most Western countries the least common birthday is December 25. For instance, in the United States between 1973 and 1999, September 16 is the most common birthday and December 25 the least common birthday. Some other uncommon birthdays in the West are January 1, December 24, and April 1.

The least common birthday is usually a holiday when most people take time off work. There are multiple reasons why these days have fewer births, but one reason is that doctors are less likely to schedule cesarean births on such days. April 1 is an exception, and is likely because parents who are able to choose not to give birth on that day, to prevent their children from being bullied for having their birthday on April Fool's Day.

This question is harder to answer in many non-Western countries as there is less easily accessible data. For example in a survey of students in India between 2007 and 2012, the statistics show much more births just before school opens, and on round days like the 5th, 10th, 15th, and 20th. This indicates much of the data may be faked.