There are two mainstream ways in which to count time; by the solar calendar directed by the movement of the sun, or the lunar calendar directed by the movement of the moon. Most of the world goes by the solar calendar. The Julian calendar created by the Greeks is directed by the movement of the sun in relationship to the stars. One year is approximately 365 ¼ days. So for 3 years we have 365 days and the fourth year we have an extra day called a leap year to account for the extra ¼ day.
Alternatively, the lunar calendar is a combination of a solar and lunar calendar based with a leap year to accommodate the discrepancy with the solar rotation. This form of counting time is used in the Jewish world. Within it, every 3 or 4 years there is a leap year with an extra month called Adar II, which follows the regular month of Adar I in a leap year. This is done so that all holidays fall into the correct seasons; year upon year.
Rosh Chodesh: The New Moon
The first day of each month is commemorated when one can hardly see any moon at all, waxing to the middle of the month when there is a full moon, and then waning towards the end of the month where one cannot see it at all again; the new moon. Rosh Chodesh is the last day of a month and the first day of the next.
Holidays and Celebrations