kal'-en-dar (Latin calendarium, "an account book," from calendae, "day on which accounts were due"): The Hebrew or Jewish calendar had three stages of development: the preexilic, or Biblical; the postexilic, or Talmudic; and the post-Talmudic. The first rested on observation merely, the second on observation coupled with calculation, and the third on calculation only. In the first period the priests determined the beginning of each month by the appearance of the new moon and the recurrence of the prescribed feasts from the vernal and autumnal equinoxes. Thus, the month Abib ('abhibh), the first month of the year according to the Levitical law, in which the Passover was to be celebrated, was determined by observation (Ex 12:2; De 16:1-22). After the exile more accurate methods of determining the months and seasons came into vogue, and calculation was employed to supplement and correct observations and the calendar was regulated according to the Babylonian system, as is evidenced by the names of the months which are derived from it. In later times the calendar was fixed by mathematical methods (see the article "Calendar" in the Jewish Encyclopedia). The difficulty of ascertaining the first day of the new moon by observation, in the early period, led to the celebration of two days, as seems to be indicated in 1Sa 20:27. We have only four names of months belonging to the pre-exilic period, and they are Phoenician. Of these Abib ('abhibh) was the first month, as already indicated, and it corresponded to Nis (nican) in the later calendar. It was the month in which the Exodus occurred and the month of the Passover (Ex 13:4; 23:15; 34:18; De 16:1).
The 2nd month of this calendar was Ziv (ziw) (1Ki 6:1,37); Ethanim ('ethanim) was the 7th (1Ki 8:2), corresponding to Tishri of the later calendar, and Bul (bul) the 8th, corresponded to Marchesvan (marcheshwan) (1Ki 6:38). There were course other month names in this old calendar, but they have not come down to us. These names refer to the aspects of the seasons: thus Abib ('abhibh) means grain in the ear, just ripening (Le 2:14; Ex 9:31); Ziv (ziw) refers to the beauty and splendor of the flowers in the spring; Ethanim ('ethanim) means perennial, probably referring to living fountains; and Bul (bul) means rain or showers, being the month when the rainy season commenced. The full calendar of months used in the postexilic period is given in a table accompanying this article. The names given in the table are not all found in the Bible, as the months are usually referred to by number, but we find Nican in Ne 2:1 and Es 3:7; Siwan in Es 8:9; Tammuz in Eze 8:4, although the term as here used refers to a Phoenician god after whom the month was named; 'Elul occurs in Ne 6:15; Kiclew (the American Standard Revised Version "chislev") in Ne 1:1 and Zec 7:1; Tebheth in Es 2:16; ShebhaT in Zec 1:7 and 'Adhar in Ezr 6:15 and several times in Est. These months were lunar and began with the new moon, but their position in regard to the seasons varied somewhat because of the intercalary month about every three years.
The year (shanah) originally began in the autumn, as appears from Ex 23:16 and Ex 34:22, where it is stated that the feast of Ingathering should be at the end of the year; the Sabbatic year began, also, in the Ex 7:1-25th month of the calendar year (Le 25:8-10), indicating that this had been the beginning of the year. This seems to have been a reckoning for civil purposes, while the year beginning with Nican was for ritual and sacred purposes. This resulted from the fact that the great feast of the Passover occurred in this month and the other feasts were regulated by this, as we see from such passages as Ex 23:14-16 and De 16:1-17. Josephus (Ant., I, iii, 3) says: "Moses appointed that Nican, which is the same with Xanthicus, should be the first month of their festivals, because he brought them out of Egypt in that month; so that this month began the year as to all solemnities they observed to the honor of God, although he preserved the original order of the months as to selling and buying and other ordinary affairs." A similar custom is still followed in Turkey, where the Mohammedan year is observed for feasts, the pilgrimage to Mecca and other sacred purposes, while the civil year begins in March O.S.
The year was composed of 12 or 13 months according as to whether it was ordinary or leap year. Intercalation is not mentioned in Scripture, but it was employed to make the lunar correspond approximately to the solar year, a month being added whenever the discrepancy of the seasons rendered it necessary. This was regulated by the priests, who had to see that the feasts were duly observed at the proper season. The intercalary month was added after the month of 'Adhar and was called the second 'Adhar (sheni, wa-'adhar, "and Adar"), and, as already indicated, was added about once in 3 years. More exactly, 4 years out of every 11 were leap years of 13 months (Jewish Encyclopedia, article "Calendar"), this being derived from the Babylonian calendar. If, on the 16th of the month Nican, the sun had not reached the vernal equinox, that month was declared to be the second 'Adhar and the following one Nican. This method, of course, was not exact and about the 4th century of our era the mathematical method was adopted. The number of days in each month was fixed, seven having 30 days, and the rest 29. When the intercalary month was added, the first 'Adhar had 30 and the second 29 days.