Did you know that the calendar we use today is called the Gregorian calendar and is based on how the sun is positioned relative to the stars? In some cultures and religions, such as Islam, people use the moon to set up their calendar and celebrate religious holidays. Ancient people had no clocks or printed calendars, which is why the Israelites and many of their neighbors had to watch for the new crescent moon to see when the new month started. The Jews have a calendar that has 12 or 13 months. Each day starts and ends with sunset, and the week lasts seven days, ending in a day of rest and celebration — the Sabbath.