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About the Jewish Calendar

Introduction
Since Biblical times the months and years of the Jewish calendar have been established by the cycles of the moon and the sun. Torah law prescribes that the months follow closely the course of the moon, from its birth each month to the next New Moon.
Hours in Judaism
An hour in halacha is calculated by taking the total time of daylight of a particular day, from sunrise until sunset, and dividing it into twelve equal parts.
The Jewish Day
While a day in the secular calendar begins and ends at midnight, a Jewish day goes from nightfall to nightfall.
The Jewish Month
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 Jewish Calendar Year
A standard Jewish year has twelve months; six twenty-nine-day months, and six thirty-day months, for a total of 354 days.
The Jewish Leap Year
The Jewish leap year has an added month so that the lunar-based Jewish year should remain aligned with the solar seasons.
On Which Days Do Jewish Holidays Begin?
Once you know the day of the week when Passover begins in any given year, you can then calculate when many (but not all) of the year’s holidays will be celebrated.