The Jewish calendar is based on lunar cycles. Towards the beginning of the moon’s cycle, it appears as a thin crescent-shaped. This distinguishes a new Jewish month. The moon cultivates until it is full; the middle of the month, and then it begins to fade until it cannot be seen. It remains unseen for approximately two(2)days—and then the thin crescent returns, and the cycle starts all over again.
The entire cycle takes approximately 29 and half days.3 Since a month needs to consist of complete days, a month is sometimes twenty-nine days long (such a month is known as chaser, "missing"), and sometimes thirty (malei, "full").
When the month begins it’s always been important in Jewish practice, because the Torah schedules the Jewish festivals according to the days of the month. The first day of the month and also the thirtieth day of a malei month, called Rosh Chodesh, the "Head of the Month," and semi-festive postions. See Why is Rosh Chodesh sometimes one day and sometimes two.