FOR the Hebrews in the Promised Land, the secular year began with plowing and sowing, which occurred in what is now September/​October.

A lunar calendar of 12 months (consisting of 29 or 30 days each) results in a year that is shorter than a true solar year. Various methods have been used to align the two. This can be done by adding extra days or by periodically inserting an additional month, perhaps before the start of the next year. Such a system coordinates the calendar with the seasons when crops are planted or harvested.

In Moses’ time, however, God told His people that the religious year was to begin with the month of Abib, or Nisan, in the spring. (Ex. 12:2; 13:4) A festival that took place in this month involved the barley harvest.​—Ex. 23:15, 16.

“The rule according to which it was decided whether to intercalate or not was very simple,” says scholar Emil Schürer, in his book The History of the Jewish People in the Age of Jesus Christ, (175 B.C.–A.D. 135). “The feast of Passover, to be celebrated at full moon in the month of Nisan (14 Nisan), must always fall after the vernal [or, spring] equinox . . . If, therefore, it was noticed towards the end of the year that Passover would fall before the vernal equinox, the intercalation of a [13th] month before Nisan was decreed.”

Jehovah’s Witnesses take this rule into consideration when determining the appropriate date for the Lord’s Evening Meal, which occurs in the spring and corresponds to Nisan 14 on the Hebrew calendar. Congregations around the globe are informed of this date in advance. a

But how did the Hebrews know when one month would end and a new month would begin? Today you might simply look at a printed calendar or a calendar app on your electronic device. However, in Bible times, it was not that simple.

At the time of the Flood, months were considered to be 30 days long. (Gen. 7:11, 24; 8:3, 4) Later, among the Hebrews the calendar month was not fixed at 30 days. For the Hebrew calendar, a month began at the visible appearance of the crescent of the new moon. That worked out to be 29 or 30 days after the start of the preceding month.

On one occasion, both David and Jonathan referred to a month, saying: “Tomorrow is the new moon.” (1 Sam. 20:5, 18) So it seems that by the 11th century B.C.E., the months were calculated in advance. How would the average Israelite be able to determine when a new month started? The Mishnah, a compilation of Jewish oral law and tradition, provides some information. It indicates that in postexilic times, the Sanhedrin (the Jewish high court) was involved. During the seven months in which festivals were held, the court met on the 30th day of the month. Those men were responsible for determining when the next month would begin. On what basis?

Men who were posted at vantage points around Jerusalem watched the night sky for the first sliver of a new moon. They would quickly alert the Sanhedrin. When the group in this court felt that they had sufficient testimony that a new moon had been sighted, they announced the start of a new month. What, though, if clouds or fog obscured the watchmen’s view, preventing them from seeing the crescent of the new moon? The month then in progress was declared  to have 30 days, and the new month could begin.

The Mishnah explains that the Sanhedrin’s decision was announced by means of a fire signal that was lit on the Mount of Olives, close to Jerusalem. At other high points throughout Israel, bonfires would be used to spread the news. In later periods, messengers were dispatched. Thus Jews in Jerusalem, throughout Israel, and in scattered communities were made aware of the start of a new month. All could then observe seasonal festivals at the same time.

You may find the accompanying chart helpful in understanding how the Hebrew months, festivals, and seasons were all interconnected.