Excerpted from Rabbi Elchanan Adler’s Yerach Tov: Birkat HaChodesh in Jewish Law & Liturgy
Chapter 2: The Mitzvah of Kiddush HaChodesh
Hashem spoke to Moshe and Aharon in the land of Egypt, saying: this month is for you the head of the months; it is the first for you of the months of the year. — Shemot 12:1
Hashem spoke to Moshe saying: speak to the Jews and tell them Hashem’s set times that you should call mikra’ei kodesh. These are My set times . . . These are Hashem’s set times that you shall call at their proper times. — Vayikra 23:1–2,4
Kiddush HaChodesh was the first commandment received by the Jews as a nation. In Egypt, Hashem instructed Moshe that “this month (Nissan) is the first month.” According to Talmudic tradition, Hashem pointed at the new moon and told Moshe: “When you see thus, sanctify the new month.”
The formal act of Kiddush HaChodesh is performed by beit din, an authorized Jewish court of law; only it can sanctify the new month. The court must be composed of sages of the highest caliber, i.e., sages who possess an unbroken tradition and permission to judge from Moshe and his court. This, too, is derived from Hashem’s instruction to Moshe, which utilized the word, “lakhem, to you,” indicating that only people of Moshe’s stature could participate in Kiddush HaChodesh. The Talmud finds allusion to this in the Musaf prayer of Rosh Chodesh, where we recite, “who sanctified Israel and Rosh Chodesh.” The blessing, according to the Talmudic tradition, thanks Hashem for imbuing Israel (through its representative body, the beit din) with the sanctity that enables it to sanctify Rosh Chodesh. Indeed, because of Israel’s exalted status, the judges’ determination of the month’s beginning is absolute; if they find it necessary for any reason, they may deliberately declare that the halakhic month begins earlier or later than the astronomical month.
You – even unintentionally; you – even deliberately; [you – even under compulsion]; you – even if misled.
All holidays fall on distinct calendar dates; hence, the court’s decision of when to sanctify the month (rather than the astronomical cycle of the moon) determines when these holidays will fall.
Rambam expands both the importance and scope of court’s role. First, he elevates it to a mitzvat aseh, a positive commandment. Second, he expands the court’s responsibility, adding that it must send out, and inform, the people what day Rosh Chodesh will be so that they will know on what days the holidays will fall:
It is a Biblical positive commandment incumbent upon the court to calculate and know whether or not the moon will be visible, and to probe the witnesses until they sanctify the month, and send out and inform the rest of the nation on what day Rosh Chodesh is, so they should know when the holidays are, as it says, “that you shall call them [the holidays] sacred callings,” and it says, “you shall keep this law in its time.”
R. Soloveitchik argues that the positive commandment has two distinct components: first, it obligates the court to declare a certain day Rosh Chodesh, for the purpose of offering the korbanot of Rosh Chodesh; second, it obligates it to declare Rosh Chodesh for the purpose of determining the holidays’ dates. The second component, relating to holidays’ dates, is what obligates the court to inform the other Jews of Rosh Chodesh’s date. R. Soloveitchik further argued that these two obligations derive from separate verses. Finally, he suggests that these two components are hinted at in the ritual itself. The court declares, “mekudash mekudash, sanctified sanctified,” to sanctify the month; one “sanctified” refers to investing Rosh Chodesh with its own sanctity, and the second refers to sanctity qua determining the holidays’ dates.
Rambam here mentions the verse of “you shall call them,” rather than the verse of “this month is for you the head of the months.” However, Rambam elsewhere does mention “this month is for you,” and cites the Rabbinic tradition that Hashem showed Moshe exactly how the new moon must appear in order for the court to declare Rosh Chodesh.
R. Soloveitchik explains that Rambam’s organization supports the aforementioned analysis. When Rambam invokes the mitzvah to sanctify the new month, he derives it from “this month is for you,” and here, where he invokes
the mitzvah to facilitate observance of the holidays in their proper time (which includes the requirement of sending messengers to inform the rest of the nation when Rosh Chodesh is), he derives it from “these are Hashem’s holidays that you shall call at their proper time.”
R. Soloveitchik also infers from the words of Rashbam that the holidays are sanctified automatically when the court declares Rosh Chodesh. However, he notes that Rabbenu Chananel holds that the courts must sanctify separately the day of Rosh Chodesh and the holidays that occur during the upcoming month.