Rosh Chodesh

Rosh Chodesh is the name for the first day of every month in the Hebrew calendar marked by the appearance of the new moon. 

The Jewish calendar is based on the lunar cycle. The beginning of the moon’s cycle appears as a thin crescent. That is the signal for the new Jewish month. The moon grows until it is full, the middle of the month, and then it begins to wane until it cannot be seen. It remains invisible for approximately two days,  then a thin crescent appears, and the cycle repeats again and again

The entire cycle takes approximately twenty-nine and a half days. Since a month needs to consist of complete days, a Jewish month is sometimes twenty-nine days long (known as chaser, “missing”), and sometimes thirty (meaning malei, “full”).

Knowing exactly when the month begins has 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, as well as the thirtieth day of a malei month, is called Rosh Chodesh, the “Head of the Month,” and has somewhat festive status

"Also on your days of rejoicing, feasts, And new moons, you are to blow on the trumpets over your burnt offerings and fellowship offerings. They will then be a reminder for you before Adonai your God. I am Adonai your God!”  

                                                                                                                                                        Numbers 10:10  (TLV)