Halakhah su Levitico 23:4

It occurred that two [witnesses] came and said, "We saw [the moon] in the morning in the east and in the evening in the west." R. Yoḥanan ben Nuri said, "They are false witnesses." When they came to Yavneh, Rabban Gamaliel accepted them. Also, two [witnesses] came and said, "We saw [the moon] in its proper time but on the following night it was not seen" and Rabban Gamaliel accepted them. R. Dosa ben Horkanos said, "They are false witnesses. How can people testify that a woman has given birth when the next day her abdomen is between her teeth?" R. Joshua said to him, "I accept your words." Rabban Gamaliel said to him, "I decree that you come to me with your staff and your money on the day on which Yom Kippur falls according to your reckoning." R. Akiva went and found [R. Joshua] in distress. [R. Akiva] said to him, "I can derive that everything Rabban Gamaliel has done is valid as it says, 'These are the appointed seasons of the Lord, holy convocations which you shall proclaim in their appointed seasons' (Leviticus 23:4), i.e., whether [they are proclaimed] at their proper times or other than at their proper time, I have no appointed seasons other than these." [R. Joshua] came to R. Dosa ben Horkanos. [R. Dosa ben Horkanos] said to him, "If we examine [the decisions of] the Bet Din of Rabban Gamaliel we must examine the decisions of every single Bet Din that has existed from the time of Moses until the present."

That is that He commanded us to sanctify (proclaim) the months (in other versions, and to calculate the months) and years. And that is the commandment of sanctifying the month. And that is His, may He be exalted, saying, "This month shall mark for you the beginning of the months" (Exodus 12:2). And the explanation (Rosh Hashanah 22a) comes [and tells us] that "this testimony is given over to them" - meaning that this commandment is not given over to each and every individual, like the Shabbat of creation, towards which every individual counts six days and rests on the seventh. [Here, it is not] that when each and every individual sees the [new] moon, he determines that today is Rosh Chodesh (the first of the month), or that he should count some Torah matter and establish the new month or look into the lateness of the Spring - or something else that is fitting to observe - and add a month. Rather, this commandment is always only done by the High Court, and only in the Land of Israel. And the sighting [of the new moon] has therefore been annulled for us today with the absence of the High Court, just like the offering of sacrifices has been annulled with the absence of the Temple. And the heretics called Karaites have referred to this and erred about it. And this is a principle that even some of the rabbis did not concede and followed them into the darkness and the shade. You should know that the calculation that we count with today, through which we know Rosh Chodesh and the holidays, is impossible to do outside of the Land. However in the absence of sages in the Land of Israel, it is possible for a court that was ordained in the Land of Israel to intercalate years and determine months outside of the Land, like Rabbi Akiva did - as is explained in the Talmud (Berakhot 63a) - yet there is a great and strong question about this. And it is known that the Great Court, however, was in the Land of Israel; and that they were the ones that determined the months and intercalated the years in ways that were passed on to them, [doing so] in their gathering together. And this is one of the great principles of the faith - only those that have a deeper knowledge know it and see it in its place. And that is that that which we count today outside of the Land with the work of intercalation that is in our hands - and say that this day is Rosh Chodesh and that day is a holiday - is not because we have determined the holiday from our [own] calculation in any way. Rather, it is because the Great Court in the Land of Israel had already determined that this day is Rosh Chodesh or a holiday. And since they said that today is Rosh Chodesh or a holiday, it is [actually] Rosh Chodesh or a holiday - whether this action of theirs was through calculation or sighting - as appears in the explanation (Rosh Hashanah 25a), "'These are the set times of the Lord [...] which you shall proclaim as sacred occasions' (Leviticus 23:4); I have no other set times besides these" - meaning to say, the ones that they say are the sacred times, even under duress, even in error, even inadvertently - as it appears in the tradition. And we indeed consider the day determined by them - meaning the inhabitants of the Land of Israel - to be Rosh Chodesh. As it is upon [their] work itself that we count and determine [it] - not upon sighting; and it is upon their calculation that we rely, and not upon our [own] calculation. Rather our calculation is just an exposition of the matter. And understand this. And I will explain to you further. If we were to assume, by way of illustration, that the [Jewish] residents of the Land of Israel disappeared from the Land of Israel - God forbid that God would do this, since He promised that He would not erase the traces of the nation [there] totally - and that there would not be a court there, nor a court outside the Land of Israel that was ordained there. [In such a case,] this calculation of ours would surely not help us at all in any way. For we may only calculate months and intercalate years outside the Land of Israel according to the conditions mentioned, as we have explained - 'for out of Zion comes forth Torah.' And when someone with a complete intellect examines the [related] statements of the Talmud with this approach, everything that we said will become clear, without a doubt. And note that there were hints that appear in Scripture that indicate the principles of this work upon which we should rely to know Rosh Chodesh and the intercalation of years. Among them is His saying, "You shall keep this ordinance at its set time from year to year" (Exodus 13:10). They said (Mekhilta DeRabbi Shimon Bar Yochai 13:9), "It teaches that we only intercalate the year during the period that is proximate to the sacred occasion (Pesach)." And they said [further], "From where [do we know] that we only sanctify the month during the day? [Hence] we learn to say, 'from year to year (literally, from day to day).'" And they [also] said (Megillah 5a), "You calculate months for years, but you do not calculate days for years." This indeed indicates that the addition in this is in fact a full month. And they said [further], "'A month of days' - you count the days of a month, and you do not count the hours of a day." And His saying, "Observe the month of Spring" (Deuteronomy 16:1), indicates that it is fitting that we preserve the seasons of the year in our years, and that is why it is [also] solar. And the regulations of this commandment have already all been completely explained in the first chapter of Sanhedrin, in Tractate Rosh Hashanah and in Berakhot. (See Parashat Bo; Mishneh Torah, Sanctification of the New Month 1.)

And there is one there that learns it from, "all work of labor shall you not do" (Leviticus 23:7) - and that is Rabbi Yose HaGalili. And this is how the verse is explained according to him - meaning, on the first day is it that all work of labor is forbidden besides what is needed for the food of the soul, but on the intermediate days of the festival, not all work is forbidden on it. Rather some are forbidden and some are permitted; and Scripture gave them over to the Sages [to decide]. And Rabbi Akiva learns it from, "These are the appointed times of the Lord, holy occasions" (Leviticus 23:4) - as he establishes it as [referring to] the intermediate days of the festival. And from that which it is written in it, "holy occasions," it teaches that it is forbidden in the doing of work. And it is possible that according to him, permitted work is learned by him from (Leviticus 23:36) "it is an atseret (which can also mean, a cessation)" - meaning that the eighth day is stopped from all work, but not the other days. Or also, he learned it form [the letter] hay (which means, the) of "the seventh," written about Pesach in the Order of Reeh Anochi (Deuteronomy 16:8), "Six days shall you eat matsot," such that they, may their memory be blessed, said "'And on the seventh day it is an atseret' - the seventh is stopped from all work, but not the sixth. And which work is forbidden or permitted? Scripture gave them over to the Sages [to decide]." And since the matter is given over to their hand, such that the Torah only forbade what they said, they divided work according to their will and opinion. And it comes out that all work they, may their memory be blessed, forbade, is forbidden to us from Torah writ; and that which they permitted is also permitted by Torah writ. As according to the understanding expounded from Scripture, this prohibition is given over to their hand.