[1] The main ideas presented here are taken from mt, Laws of Sanctifying the Month 5:1-3. In Sefer Ha-mitzvot, Positive Commandment 153, Rambam writes that the sanctification of the months is entrusted to the Beit Din Ha-gadol. Ramban, however, claims that a beit din of three will suffice. It may be that there is no argument here, as Rambam means to say that the months are sanctified merely by virtue of the foremost beit din of the generation, not that they actually must sanctify the months in practice.

As stated above, the months were sanctified primarily in Eretz Yisrael, and only in dire circumstances did the greatest rabbis of the generation, who were ordained in Eretz Yisrael, leave the Land in order to sanctify the months without interference from the kingdom. This is derived from a passage in Berakhot 63a:

When Ĥanina, R. Yehoshua’s nephew, went down to the Diaspora (after the Betar revolt), he intercalated years and established months outside Eretz Yisrael. The rabbis of Eretz Yisrael sent two Torah scholars after him: R. Yossi b. Kipar and the grandson of Zekharia b. Kevutal. When he saw them, he said, "Why have you come?" They replied, "We have come to learn Torah." He proclaimed, "These men are among the greatest rabbis of the generation, and their forefathers served in the Holy Temple." He began declaring things impure while they declared them pure. He forbade certain acts and they permitted them. So he announced, "These men are worthless; they are empty." They said to him, "You already built us up; you can no longer knock us down. You already built a fence; you can no longer breach it." He responded, "Why is it that whenever I declare something impure you declare it pure; whenever I forbid something you permit it?" They answered, "Because you are intercalating years and establishing months outside Eretz Yisrael." He said, "Didn’t Akiva b. Yosef intercalate years and establish months outside Eretz Yisrael?" They replied, "Leave aside the case of R. Akiva, for he did not leave behind anyone like him in Eretz Yisrael." He said to them, "I, too, left no one behind like me in Eretz Yisrael." They answered, "The kids you left behind have become goats with horns (i.e., the young students you left behind have become great Torah scholars in their own right), and they sent us to you, and this is what they told us: Go, tell him in our names to desist. If he listens – fine. But if he does not – excommunicate him. And tell our brethren in the Diaspora to desist as well. If they listen – fine. But if not – let them go up to the mountain, where Aĥiya will build an altar and Ĥanania will play the harp, and everyone will deny God, saying, ‘We have no portion in the God of Israel.’" Upon hearing this, the entire congregation immediately burst into tears and said, "God forbid! We do have a portion in the God of Israel." And why all the fuss [not to establish the calendar outside Eretz Yisrael]? Because it says, "For Torah shall come forth from Zion, the word of the Lord from Jerusalem" (Yeshayahu 2:3).

We stated above that when semikha does not exist, the months are sanctified by way of mathematical calculations. According to Rambam, this is a halakha le-Moshe mi-Sinai (a law that God taught Moshe at Mount Sinai). This is why he does not mention Hillel ii in his discussion on the topic. Ramban, on the other hand, writes that there is no source indicating that this is a halakha le-Moshe mi-Sinai. Rather, the truth is that in our times as well, the months are considered sanctified by rabbis who have undergone semikha – that is, based on Hillel ii’s calculations. Consequently, Ramban does not require the existence of Jews living in Eretz Yisrael for the calculations to apply to them, as he maintains that the months are not established based on the calculations of those currently living in Eretz Yisrael, but rather upon Hillel ii’s ancient calculations (see ahs 417:7).