The calendar is a systematized account of the passage of time and is used for the chronological organization of activities. The origin of the Jewish calendar cannot be established with any certainty. In Exodus 12:2 it is written: “This month shall be for you the beginning of months. It shall be for you the first month of the year.” This verse is one of the earliest mentions involving the use of a calendar.

The Jewish calendar in its current form is known to have existed for 1,600 years. Previously, the Sanhedrin was in charge of defining the calendar through a council called Sod-ha-ibur, which literally means “secret of the calendar intercalation.” But it was in the year 359 CE that the meetings of the Sanhedrin were banned by the Roman Emperor Constantius II and in order to prevent the Jewish people from being left without a calendar, Hillel II, Nasi of the Sanhedrin, decided to make public the secrets of its structure.

The Jewish calendar is calculated according to two cycles: the lunar and the solar. This is because the days and dates of religious ceremonies are determined by the phases of the moon, while the seasons are based on the revolution of the earth around the sun. This dual system was necessary because the twelve-month lunar year is approximately eleven days shorter than the solar year. If this difference were not corrected, over the years the festivals that form the backbone of Jewish history would shift from season to season, altering their character. Passover, the spring festival, might have fallen in winter.

The problem was solved by inserting an extra month in some years, thus making the festivals coincide with the corresponding season. In a 19-year cycle, the Jewish calendar has seven years that have 13 months instead of 12. This extra month, Adar Bet, is added to years 3, 6, 8, 11, 14, 17, and 19. Also, since a lunar month lasts 29 days, 12 hours, and 44 minutes, some months on the Jewish calendar have 29 days and others 30, depending on the type of year in question.

In ancient times, the months were named only by their numerical order, starting in the northern spring with the first month. Later, the names of the Hebrew months were taken from the Babylonian calendar. For example, the month of Tishrei was called “Tashritu” in Babylonian.

The counting of the Jewish years, as we know it today, dates back to the Middle Ages. Our sages defined the Creation of the World as the starting point for counting the years. The calculation is based on the Bible, on the lists of the generations that followed Adam and Noah and his sons (Chapters 5, 10 and 11 of Breishit) and on the various historical events. Thus, Rabbi Yossi ben Halafta calculated that the world was created in the year 3761 BCE (Before the Common Era) and Maimonides agreed with him. Let’s do the math: 3761 years plus 2014 years of the Common Era gives us 5775, which is the current Jewish year. Thanks to our sages, we do know what day we live in.

Prepared by Marcos Gojman.

Bibliography: The Beginning of the Jewish Calendar by Bernard Dickman, Development of the Hebrew calendar by John Lemley and Counting the years by Rabbi Rachel M. Solomin.