Thirty days have September,
April, June and November.
All the rest have thirty-one,
Except February the only one
Which Leap Years change
each fourth time
From twenty-eight to twenty-nine.
Century 100s don’t always leap,
each 400 years that leap we keep
The above poem actually describes which months of the year have 31 days and which have 30 days. February is the one that has either 28 or 29 days, depending on whether the year in question is a leap or common year. To determine if a year is a leap year or a common one, we carry out the following algorithm:
Let A = Year in question
EITHER A is divisible by 4
-------AND A is not divisible by 100 => leap year
OR A is divisible by 400 => leap year
ELSE, A => common year
To summarise, a leap year is a year which is divisible by 4 and not divisible by 100, or a year which is divisible by 400. A leap year would mean that the month of February will have 29 days instead of the usual 28.