\begin{definition}[Definition:Calendar/Jewish]
The '''Jewish calendar''' is a calendar based on both the phases of the moon and on the orbital period of earth around the sun.
The whose period is based on $12$ full cycles of phases of the moon.
The basic Jewish year is $354$ or $355$ days of $12$ months, each alternately $29$ and $30$ days long.
When the error between the Jewish year and tropical year becomes approximately $30$ days, a $13$th month is inerted into the year.
\end{definition}