Rambam - 1 Chapter a Day

The Ramban [in his gloss on Sefer HaMitzvot (positive commandment 153)] differs and explains that although it was necessary for the judges who established the calendar to have semichah, there was no necessity that this function be carried out by the Sanhedrin or a court deputized by it.

This explanation was recorded for posterity in Rosh HaShanah 21b.

I.e., to the Sages of the Sanhedrin, who preserve the tradition of the Jewish court as initiated by Moses and Aaron.

In the Introduction to his Commentary to the Mishnah, the Rambam defines a halachah communicated to Moses on [Mount] Sinai as a law that is neither explicitly stated nor alluded to in the Written Law, but communicated through the Oral Tradition.
See also the statements of the Ramban (loc. cit.), who objects to the Rambam’s statements, and asks which source states that this is a halachah communicated to Moses on Mount Sinai.

As explained in Chapter 6, the fixed calendar is based on the mean motion of the sun and the moon—i.e., their average rate of progress in the heavens. As explained in Chapter 11 onwards, the progress of the sun, and to an even greater extent that of the moon, deviates from this mean from time to time. Although ultimately these deviations will balance out, at any given time the day established as Rosh Chodesh may be before or after the day on which the moon can be sighted.

As explained in Chapter 18, Halachah 13, in the lands west of Eretz Yisrael the moon is sighted earlier than in Eretz Yisrael.

Based on the Rambam’s statements in Sefer HaMitzvot (loc. cit.), the Chatam Sofer (Yoreh De’ah, Responsum 234) explains that the Rambam’s intent is that, from that time onward, a formal court no longer held sessions in Eretz Yisrael; the land, however, remained populated by Jews. (See the notes on Halachah 5.)

This concept is derived from Rosh HaShanah 21a, which states that Ravvah fasted two days on Yom Kippur, because he did not know which day was established by the court in Eretz Yisrael.

This refers to the establishment of the fixed calendar by Rabbi Hillel (Hillel II), approximately two generations after the composition of the Jerusalem Talmud.

See Chapter 3, Halachot 11-13.

Implicit in the Rambam’s words is a concept explained in greater detail in Halachah 13: The determination of the calendar depends on the establishment of the calendar by the court in Eretz Yisrael.

As prescribed by the Torah. Once the date on which the festivals should be celebrated could be universally known because of the fixed calendar, the safeguard of celebrating the holidays for two days was no longer necessary.

Beitzah 4b explains that this rule was ordained lest the gentile authorities oppress the Jewish people and prevent the inhabitants of the Diaspora from communicating the tradition of the calendar to their descendants. They would then have to rely on information conveyed from Eretz Yisrael again.

See, however, Halachot 11 and 12.

I.e., originally, the second day of the holidays was observed because of the doubt of the day on which to observe the Torah’s commandment. In the present era there is no longer any doubt, and the observance of the second day of a holiday has the status of a Rabbinic ordinance.

Generally, the court would endeavor to structure the calendar so that the moon would be sighted on the thirtieth night of Elul. In expectation of the probability that the following day would be declared Rosh HaShanah, the people would observe all the holiday prohibitions and sound the shofar on the thirtieth day. Nevertheless, since it was possible that the moon had not been sighted, the people outside the immediate surroundings of Jerusalem would observe the following day as well.

See Chapter 3, Halachah 8.

As mentioned above, except for the offering of the Rosh HaShanah sacrifices, the thirtieth day following Rosh Chodesh Elul was always observed as Rosh HaShanah in expectation of the arrival of witnesses.

For it would actually be observed as Rosh HaShanah.

Including even the inhabitants of Jerusalem.

As the Rambam continues to explain, the determining factor is not the geographic distance of a place from Jerusalem, but rather the custom followed in this place at the time when the High Court sent out messengers.

See Bava Batra 38a and Ketubot 17b, which tell of a time when a blockade prevented communication between Jerusalem and the Galilee.

For the messengers were forbidden to travel on Rosh HaShanah, Yom Kippur, and the two Sabbaths between Rosh HaShanah and Sukkot. In the years when Rosh HaShanah is observed on the Sabbath, the messengers could proceed a further day. Nevertheless, since this is not so in the majority of the years, a ten-day journey is accepted as the standard measure.

Note Hilchot Terumah 1:3, which defines Syria as referring to lands conquered by King David before the entire territory of Eretz Yisrael proper had been conquered. Therefore, it was not considered part of the Holy Land with regard to most ritual questions according to the Torah itself. Nevertheless, the Rabbis extended many of the laws applying to Eretz Yisrael to this territory as well.

The inclusion of Ashkelon, which is located to the southwest of Eretz Yisrael, in Syria clearly indicates that the Rambam is referring to the halachic conception of the latter term, and not the geographic one, since Syria is to the northeast of Eretz Yisrael.

Based on this halachah, according to the Rambam, most Jews living in Eretz Yisrael today should celebrate the holidays for two days. Indeed, the Eretz Tzvi, a text that documents the prevailing customs in Eretz Yisrael before the waves of European immigration, states that in cities like Ramlah, that did not have an existing custom dating back to the Talmudic period, the holidays would be celebrated for two days, as the Rambam states. It was not until the resettlement of Eretz Yisrael in the present era that such cities began observing the holidays for only one day. The source for this practice is the commentary of the Ritva (Rosh HaShanah 16b), who states that, in the present era, it is the geographic definitions of Eretz Yisrael and the Diaspora that determine whether or not one should celebrate one day.

Until the time of Rabbi Hillel, the court of Eretz Yisrael determined the calendar through their independent calculations. Even after the court ceased to function and the fixed calendar was adopted, the determination of the calendar still depends on the establishment of the calendar in Eretz Yisrael. The only difference is that from the time of Rabbi Hillel onward, instead of determining the calendar independently, the inhabitants of Eretz Yisrael would rely on the fixed calendar. For this reason, the Rambam points to a Divine covenant insuring that there will always be Jews living in our Holy Land. For were there not to be any Jews living in Eretz Yisrael, heaven forbid, we would not be able to determine the calendar.