The sighting of the Rosh Chodesh, or new moon, marks the beginning of a new month. This regulates the time of Shabbat, the Sabbath Day, and the dates of festivals, holy days, and fast days.
A day begins at sunset and ends at the next sunset. The week of seven days is believed to be based on the creation story in Genesis, not on astronomy. The calendar day is twenty-four hours long, measured from 6 p.m. to 6 p.m. in Jerusalem.
The calendar synchronizes a cycle of lunar years with solar years. In each cycle of nineteen years, there are twelve common years and seven leap years. The number of days can range from 353 to 385 because of the influence of religious requirements. There are three types of both common and leap years: regular (354 days and 384 days), deficient (353 days and 383 days), and abundant (355 days and 385 days).
There is a grouping of seven years, with the seventh being a Sabbatical year. There is also a grouping of seven of these groups, ending with a Jubilee year. A rough conversion from the Gregorian calendar to the Hebrew calendar is subtract 1240 from the Gregorian and add 5000.