Hebrew Calendar. The Israelites used such a lunisolar, or bound solar, calendar. This is evident from the fact that Jehovah God established the beginning of their sacred year with the month Abib in the spring and specified the celebration of certain festivals on fixed dates, festivals that were related to harvest seasons. For these dates to have coincided with the particular harvests, there had to be a calendar arrangement that would synchronize with the seasons by compensating for the difference between the lunar and solar years.​—Ex 12:1-14; 23:15, 16; Le 23:4-16.

The Bible does not indicate what method was originally used to determine when additional days or an additional, or intercalary, month should be inserted. It is logical, however, that either the vernal or the autumnal equinox served as a guide to indicate when the seasons were falling behind sufficiently to require calendar adjustment. Though not specifically mentioned in the Bible, a 13th month that was added by the Israelites to accomplish this adjustment was called, in postexilic times, Veadar, or the second Adar.

We do not find record of a definitely fixed or standardized form of Jewish calendar until the fourth century of our Common Era (c. 359 C.E.), when Hillel II specified that the leap years of 13 months should be the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th of each 19 years. Such a 19-year cycle is commonly called the Metonic cycle, after the Greek mathematician Meton (of the fifth century B.C.E.), although there is also evidence that such a cycle was perfected before him by the Babylonians. (See Babylonian Chronology, 626 B.C.–A.D. 75, by R. A. Parker and W. H. Dubberstein, 1971, pp. 1, 3, 6.) This cycle takes into account that every 19 years the new and the full moons fall again on the same days of the solar year.

The Jewish months ran from new moon to new moon. (Isa 66:23) Thus, one Hebrew word, choʹdhesh, rendered “month” (Ge 7:11) or “new moon” (1Sa 20:27), is related to cha·dhashʹ, meaning “new.” Another word for month, yeʹrach, is rendered “lunar month.” (1Ki 6:38) In later periods, fire signals were used or messengers were dispatched to advise the people of the beginning of the new month.

In the Bible the individual months are usually designated simply by numbering according to their position in the year, from the 1st through to the 12th. (Jos 4:19; Nu 9:11; 2Ch 15:10; Jer 52:6; Nu 33:38; Eze 8:1; Le 16:29; 1Ki 12:32; Ezr 10:9; 2Ki 25:1; De 1:3; Jer 52:31) Only four months are named prior to the exile in Babylon, namely, Abib, the first month (Ex 13:4); Ziv, the second (1Ki 6:37); Ethanim, the seventh (1Ki 8:2); and Bul, the eighth (1Ki 6:38). The meanings of these names are strictly seasonal, thus giving additional proof of a lunisolar year.​—See the individual months by name.

In postexilic times the names of the months used in Babylon were employed by the Israelites, and seven of these are mentioned: Nisan, the 1st month, replacing Abib (Es 3:7); Sivan, the 3rd month (Es 8:9); Elul, the 6th (Ne 6:15); Chislev, the 9th (Zec 7:1); Tebeth, the 10th (Es 2:16); Shebat, the 11th (Zec 1:7); and Adar, the 12th (Ezr 6:15).

The postexilic names of the remaining five months appear in the Jewish Talmud and other works. They are Iyyar, the 2nd month; Tammuz, the 4th; Ab, the 5th; Tishri, the 7th; and Heshvan, the 8th. The 13th month, which was intercalated periodically, was named Veadar, or the second Adar.

Eventually the length of most of the months was fixed as having a specific number of days. Nisan (Abib), Sivan, Ab, Tishri (Ethanim), and Shebat regularly had 30 days each; Iyyar (Ziv), Tammuz, Elul, and Tebeth regularly had 29 days each. Heshvan (Bul), Chislev, and Adar, however, could have either 29 or 30 days. The variations in these latter months served to make necessary adjustments with the lunar calendar but also were used to prevent certain festivals from occurring on days viewed as prohibited by later Jewish religious leaders.

Whereas the sacred year began in the spring with the month Abib (or Nisan) by God’s decree at the time of the Exodus (Ex 12:2; 13:4), the Bible record indicates that prior to this the Israelites had counted the year as running from fall to fall. God gave recognition to this arrangement so that, in effect, there was a dual system of a sacred and a secular or agricultural calendar used by his people. (Ex 23:16; 34:22; Le 23:34; De 16:13) In postexilic times, Tishri 1, in the last half of the year, marked the beginning of the secular year, and the Jewish New Year, or Rosh Hashanah (head of the year), is still celebrated on that date.

In 1908 the only approximation of an ancient written Hebrew calendar was found at the site of Gezer, and it is believed to be from the tenth century B.C.E. It is an agricultural calendar and describes agricultural activity beginning with the autumn. In brief, it describes two months each of storage, sowing, and spring growth, followed by one month each of pulling up flax, barley harvest, and a general harvest, then two months of pruning the vines and, finally, one month of summer fruit.​—Le 26:5.

The chart accompanying this article shows the months in their relation to both the sacred and secular calendars and also their approximate correspondence to the months of our present calendar.

The frequent references in the Gospel accounts and the book of Acts to the various festival seasons show that the Jewish calendar continued to be observed by the Jews during the time of Jesus and the apostles. These festival seasons serve as a guide for determining the relative time of the Biblical events of that day.​—Mt 26:2; Mr 14:1; Lu 22:1; Joh 2:13, 23; 5:1; 6:4; 7:2, 37; 10:22; 11:55; Ac 2:1; 12:3, 4; 20:6, 16; 27:9.