Leap year vs common year
A leap year (or intercalary year) consists of 366 days instead of 365 in common one. In each leap year, the month of February has 29 days instead of 28.
Everyone knows that years whose division by 4 equals an integer are leap years.
But there is an exception to this rule: every 100th year is not leap year, except every 400th year, which is leap. E.g. 2100 will not be leap year, but 2000 was leap.
To help you determine whether a year is leap or not, I've made this simple calculator. Just enter a year and get number of days in this year: