Seven days form a week. The days of the week are described in the Bible and the Talmud simply as the first day, the second day, &c. Only the seventh day has a second name, Yom ha-shabbath or shabbath, “the day of rest,” or “the rest.” In post-Biblical literature the sixth day is called Erebh shabbath or Ma’ale shabbatha, “the eve of Sabbath,” or “the coming in of Sabbath.” The evening following Sabbath is named Motseë shabbath, “the departure of Sabbath.” Similarly the day preceding a Festival and the evening following it are called Erev yom-tobh and Motseë yom-tobh, “the eve of the Festival.” and “the departure of the Festival.” Four weeks and one or two days make one month, ‏חדש‎ or ‏ירח‎. The length of the month is determined by the duration of one revolution of the moon round the earth. Such revolution is completed in twenty-nine days and a half. [77] As, however, the calendar mouth does not commence in the middle of the day, but at the beginning of the evening, it was necessary to add half a day to one month, and to take off half a day from the next. The months have therefore alternately twenty-nine and thirty days. The months are named according to their order, the first month, the second, &c.; the first being the first month in the spring. Other names, implying agricultural and climatic relations, were likewise in use, and the following four of them have been preserved in the Bible: the first month is called Abib, “ears of corn;” the second Ziv, “beauty;” the seventh Ethanim, “hardy fruit;” and the eighth, Bul, “rain.” [78] Since the return of the Jews from the Babylonian exile, names of foreign origin have been in use, viz., Nisan, Iyar, Sivan, Tammuz, Abh, Elul, Tishri, Cheshvan, Kislev, Tebheth, Shebhat and Adar. [79] Roughly speaking, these months correspond to April, May, June, July, August, September, October, November, December, January, February, and March. The year is either an ordinary year or a leap-year, the former consisting of twelve, the latter of thirteen months. The extra month is called Adar-sheni, “the second Adar,” and is added between Adar and Nisan. It serves to adjust from time to time the lunar to the solar year; [80] for there is between the lunar year—that is, the time of twelve revolutions of the moon round the earth—and the solar year, or the time of one revolution of the earth round the sun, a difference of about eleven days, the one consisting of about 354⅓, the other of about 365¼ days. In nineteen years the difference amounts to about seven months. We have therefore seven leap-years in every cycle (‏מחזור‎) of nineteen years, viz., the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th.