Definition:Leap Year

Definition
The number of days in a year is not an integer, but lies somewhere between $365$ and $366$.

However, it is convenient to assign an integer number of days to any given year.

It is also desirable to ensure that a particular day of the year occurs at approximately the same time each year.

In the Julian and Gregorian calendars:
 * some years are assigned $365$ days

and:
 * some years are assigned $366$ days.

The years which have $366$ days are called leap years.