Since the lunar month always began with the appearing of the new moon (Heb., choʹdhesh), the term “new moon” also came to mean “month.” (Ge 7:11; Ex 12:2; Isa 66:23)

Since the average lunation from new moon to new moon is about 29 days, 12 hours, 44 minutes in length, the ancient lunar months had either 29 or 30 days. This may originally have been determined by simple observation of the appearance of the new moon’s crescent; but in David’s time we find evidence of its being calculated beforehand. (1Sa 20:5, 18, 24-29) Nevertheless, in postexilic times the Mishnah (Rosh Ha-Shanah 1:3–2:7) states that the Jewish Sanhedrin met early in the morning on the 30th day of each of seven months in the year to determine the time of the new moon. Watchmen were posted on high vantage points around Jerusalem and carried immediate report to the Jewish court after sighting the new moon. Upon receiving sufficient testimony, the court announced, ‘It is consecrated,’ officially marking the start of a new month. If cloudy skies or fog caused poor visibility, then the preceding month was declared to have had 30 days, and the new month began on the day following the court assembly. It is also said that further announcement was made by a signal fire lit on the Mount of Olives, which was then repeated on other high points throughout the country. This method was evidently replaced later by the dispatching of messengers to carry the news.

In the fourth century of our Common Era a standardized or continuous calendar was established so that the Jewish months came to have a fixed number of days, with the exception of Heshvan and Chislev as well as the month Adar, which still vary between 29 and 30 days according to certain calculations.

New Moon Observance. Among the Jews each new moon marked the occasion for the blowing of trumpets and the offering up of sacrifices according to the Law covenant. (Nu 10:10; 2Ch 2:4; Ps 81:3; compare Isa 1:13, 14.) The offerings prescribed were, in fact, even greater than those normally offered on the regular Sabbath days. (Nu 28:9-15) While nothing is stated specifically as to the new moon’s marking a day of rest, the text at Amos 8:5 indicates a cessation of labor. It was apparently a time of feasting (1Sa 20:5) as well as an opportune time to gather for instruction in God’s law.​—Eze 46:1-3; 2Ki 4:22, 23; Isa 66:23.

The seventh new moon of each year (corresponding to the first day of the month of Ethanim, or Tishri) was sabbatical, and the Law covenant decreed it to be a time of complete rest. (Le 23:24, 25; Nu 29:1-6) It was the “day of the trumpet blast,” but in a greater sense than that of the other new moons. It announced the approach of Atonement Day, held on the tenth day of the same month.​—Le 23:27, 28; Nu 29:1, 7-11.