\begin{definition}[Definition:Leap Year]
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'''.
\end{definition}