Zmanim · The calendar and the moon
How does the Hebrew calendar work?
The Hebrew calendar is lunisolar: its months follow the moon, running twenty-nine or thirty days each, while a thirteenth month is added seven times in every nineteen-year cycle so that the lunar year keeps pace with the sun. That extra month is what keeps Passover, in Hebrew Pesach, always falling in the spring.
The moon and the sun, held together
A purely lunar year, twelve moons, runs about eleven days short of the solar year. Left alone, the festivals would drift backward through the seasons, and within a few years Pesach would wander out of the spring the Torah ties it to. The Jewish calendar refuses that drift. It keeps the months faithful to the moon, so each new month begins near the new moon and runs twenty-nine or thirty days, and it keeps the year faithful to the sun by adding a whole extra month, a second Adar, in seven years out of every nineteen. Two clocks that would pull apart are bound into one.
The source
The rhythm of the months and the witnessing of the new moon are worked out in the Talmud, Rosh Hashanah 25a, and the full reckoning of the calendar, the calculation that fixes months and leap years, is laid out by Maimonides in his Hilchot Kiddush HaChodesh, the Laws of Sanctifying the New Month. Together they carry a system old enough to have once depended on witnesses climbing to testify that they had seen the new moon.
Why the leap month matters
The seven leap years in nineteen are not a technicality; they are how the people keeps its word to the seasons. Because of them, Pesach is always a spring festival, Sukkot always meets the autumn, and the holidays land where the Torah placed them rather than sliding loose. When you see a Jewish year with two months of Adar, you are watching this correction happen.
What to do
You do not need to compute this by hand, but it helps to know what you are reading. When a date looks 'early' or 'late' against the secular calendar, that is the moon and the sun being reconciled. Follow a reliable Hebrew calendar for your dates, and in a leap year notice the added Adar for what it is: the hinge that keeps the whole year true.
Source: Talmud, Rosh Hashanah 25a; Rambam, Hilchot Kiddush HaChodesh.
When you want the Hebrew date and its moments for the day in front of you, Kimá (the Jewish clock: your city's zmanim and candle times) keeps the lunisolar calendar and your city's times in one place, so the ancient reckoning meets your actual week.
Open Kimá