February 29

February 29 is leap day in the Gregorian Calendar and generally occurs every 4 years. The rule for leap year is that it is in most years that are divisible by 4 but not in years that are also divisible by 100 (with the exception of years that are divisible by 400).  

For example, 2016, 2020, 2024, 2028 (all divisible by 4 but not 100) are leap years.
2100, 2200, 2300 are not leap years since they are divisible by 100 while 2400 is a leap year even though it is divisible by 100 because it is also divisble by 400.