- Peninei Halakha
As a result of Roman persecution, Jewish settlement in the Holy Land dwindled during the talmudic period, while the larger community of Jews in Babylonia advanced in all areas. Nonetheless, the Sages of Eretz Yisrael retained the authority to sanctify months and intercalate years, as it says, "For Torah shall come forth from Zion" (Yeshayahu 2:3). They would dispatch messengers every month to inform the Jews residing in the Diaspora when the new month began. Only on rare occasions, like during the Betar revolt, when the situation in the Holy Land was unbearable and the Sages were unable to sanctify the months, did they deviate from this procedure. During such times, judges ordained in Eretz Yisrael would leave the Land and travel to places that were free of such persecution and establish the order of the months and the years there.
Eventually, the Roman decrees intensified, and, as a result of Christian influence, targeted the Sages in an attempt to stop them from sanctifying the months. During this period, the rabbis sometimes would have to sanctify the month while in hiding and inform the Sages of Babylonia by sending them a secret letter (see San. 12a).
Toward the end of the talmudic period, Hillel ii came to the conclusion that the batei din of Eretz Yisrael would no longer be able to sanctify the months. In addition, there were fears that hardships and evil decrees would lead to the termination of the institution of semikha. Since Hillel ii, as the nasi (president) of the Beit Din Ha-gadol – having inherited the position in a generational chain from R. Yehuda Ha-Nasi – had the authority to set the calendar, he and his fellow judges took the initiative and calculated the months and years until the end of time and consecrated them. Thus, in the year 4119 of the Hebrew calendar (359 CE), the Jewish people began to count the months according to the Jewish calendar that Hillel ii established. We pray that we will soon be privileged to witness the final redemption, when the beit din in Jerusalem will once again sanctify the months.
Rambam postulates the very novel idea that the sanctification of the months depends on the inhabitants of Eretz Yisrael, even after semikha was abolished. That is, when the Jews of the Holy Land calculate the order of the months based on the fixed formula used to establish the calendar, then the months are considered sanctified. If, however, no Jews lived in Eretz Yisrael, God forbid, the order of the months, along with all the festivals, would cease to exist. But far be it from God to act thus, for He has promised us in the Torah that the remnants of our nation will never be destroyed.
 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).