The Hebrew calendar: from witnesses of the crescent to a fixed calculation
History of the Hebrew calendar: witnesses of the new Moon, the molad, the postponement rules, Adar I and Adar II and the nineteen-year cycle.
The Hebrew calendar is today one of the few lunisolar calendars still fully in use in religious life. Its months follow the Moon, its years remain tied to the seasons, and its main festivals always fall in the same part of the solar year. Behind this apparent simplicity lies a long history, running from direct observation of the crescent by witnesses to a system of calculation so precise that it can be applied for centuries without looking at the sky.
Witnesses before the court
In the Second Temple period and in the first centuries after its destruction, the start of the month, Rosh Chodesh, was proclaimed by the rabbinic court after it had questioned witnesses who declared that they had seen the new Moon. The Mishnah tractate Rosh Hashanah describes this procedure carefully: how the witnesses were received, what questions were asked about the shape and position of the crescent, and how the news was then passed on to distant communities.
According to the Mishnah, the announcement originally travelled from hill to hill by fires lit in sequence, until the Samaritans began lighting false fires to confuse the dates; messengers were then sent instead. For communities of the diaspora, which could not receive the news in time, the custom arose of celebrating two days of festival instead of one, a practice that still survives today outside the Land of Israel.
The calculated calendar
As rabbinic institutions in the Land of Israel declined, a fully calculated calendar took over. Tradition credits its publication to the patriarch Hillel II, around AD 359, but historians believe the present rules took shape gradually and were stable only between the ninth and tenth centuries, as shown by the famous controversy of 921-922 between Saadia Gaon and the Palestinian scholar Ben Meir over the dates of the festivals.
The heart of the system is the molad, the mean conjunction of the Moon, calculated with a month of 29 days, 12 hours and 793 parts, each hour being divided into 1080 parts. Certain postponement rules, the dechiyot, prevent the New Year, Rosh Hashanah, from falling on a Sunday, a Wednesday or a Friday, so that Yom Kippur does not fall right next to the Sabbath and other observances do not clash with the weekly day of rest.
Common years and leap years
To stay in step with the Sun, the calendar uses the nineteen-year cycle: in seven of those years, the 3rd, 6th, 8th, 11th, 14th, 17th and 19th, a thirteenth month is added. The added month is Adar: in leap years there are therefore Adar I and Adar II, and the festival of Purim is celebrated in the second. In this way Passover, fixed on 15 Nisan, always falls in spring, between late March and late April.
Years are counted from the creation of the world according to the traditional reckoning: the autumn of 2026 opened the year 5787. The months bear names of Babylonian origin, adopted after the exile, while the oldest books of the Hebrew Bible often give them an ordinal number or more archaic names, such as Aviv, the month of ears of grain, or Ethanim, the month in which the First Book of Kings places the dedication of Solomon's Temple.
The great medieval codifier Moses Maimonides devoted an entire treatise of his Mishneh Torah to the subject, the Laws of the Sanctification of the New Moon, in which he sets out both the fixed calculation and the astronomy needed to predict the visibility of the crescent. That text, written in the twelfth century, shows how far the Moon was, for Jewish tradition, not an object of worship but a sacred measure of time, entrusted to human responsibility.