11.1. The Commandment to Establish a Calendar (12:1-2)
(1) The Lord said to Moses and Aaron in the land of Egypt:
(2) This month shall mark for you the beginning of the months; it shall be the first of the months of the year for you.
(1) The Lord said: God addresses Moses and Aaron on the first day of the month of Nisan, two weeks before the Exodus.
In every society the calendar and its system of holidays create a shared memory and a set of common values. They are thus a highly essential element of communal identity. Therefore, the very first commandments that the Jewish people receive as a nation are the commandments to establish a calendar and to celebrate Passover.
The shift from the Egyptian calendar to a new, Jewish one completes the Jews’ transformation to a people entirely distinct from Egypt, and they can now see themselves as an entirely separate and independent people.
To Moses and Aaron: This revelation is addressed to Moses and Aaron together, because the new calendar is not established by fiat from Above, but by mutual consent of God and the Jewish nation together. Moses and Aaron thus appear here as representatives of God and the people, respectively.
Every month God arranges for a new moon to occur, and the people decide (that is, the Sanhedrin on behalf of the people determines) on which day each new month begins, and in which years it is necessary to intercalate an additional, thirteenth month.
In the land of Egypt: The giving of the Torah and its commandments begins not at Sinai, but in Egypt, at the onset of the Exodus.
(2) This month: The word zeh, which means “this” (or “such”), always indicates something that is clearly evident and visible. Here too, it refers to the appearance of the thin crescent of the new moon that is visible on the first evening following each monthly lunation.
For you: Establishing the months of the calendar will to some extent be entrusted to you, the Sanhedrin, who are empowered to adjust the day on which each new month begins. The Jewish months (and thus, the festivals) are very different in this respect from the Sabbath and its observance, which are determined simply by an endless cycle of seven-day intervals that never depends on any human decision, proclamation, or action.
The beginning of the months: The months are determined by the moon. A new month begins when the crescent of the new moon first becomes visible, i.e., shortly after lunation.
It shall be the first of the months of the year for you: The Jewish year begins with Nisan, the month in which the Jews left Egypt, which occurs near the beginning of spring. Thus, the Jewish calendar shifts the beginning of the year from autumn, as was then customary in the entire Middle East, to springtime. This shift reflects a decidedly optimistic approach to life and to the world – the sense of a new beginning, the opening of a new path for humanity.
Of the months of the year: Jewish tradition understands “months of the year” (emphasis on “year”) to mean that the cycle of the months must always be kept aligned with the seasons. That is, although the months are determined by the moon, the annual solar cycle must also be observed, such that the year always begins in the spring, as determined by the sun.
Thus, the calendar that the Jews received before the Exodus differed in two essential points from the calendars of the surrounding nations: It is lunar-solar, and its years always begin in the spring. The lunar-solar structure of the Jewish calendar reflects the Jewish emphasis on unifying Egyptian and Babylonian spiritual values. And having the year always begin in springtime demonstrates the optimistic nature of the Jewish worldview.
 More precisely: After the Sanhedrin acknowledges having received reliable testimony that the lunation has occurred.
 The Jewish calendar thus differs significantly from a purely lunar calendar – the Islamic calendar, for example. A “year” in a purely lunar calendar consists, simply and invariably, of twelve lunar months. And because such a “year” is approximately 11 days shorter than a true solar year, and has no connection whatsoever with actual solar cycles, the new year in such a purely lunar calendar can begin at any time during any of the four seasons.
 A year in the calendar of Tanakh always begins in springtime, and all chronology is either calculated from the year of the Exodus, or locally according to the years of tenure of the then reigning king, which are likewise numbered from Nisan (Mishnah, Rosh Hashanah 1:1). The transition from that system to the system of Jewish chronology in use today occurred at the end of the Talmudic period. Under the current system, Jewish chronology numbers its years from the Creation, and, accordingly, the count of Jewish years increases by one with each passing Rosh Hashanah, which is always observed in the autumn.