Halakhah for Exodus 13:10

The commandment of sanctifying the new month To sanctify months and intercalate [months into] years in the court [whose members are the] great in wisdom and ordained in the Land of Israel, and to establish the year's holidays according to that sanctification; as it is stated (Exodus 12:2), "This month shall be to you the first of months." This means, when you see the renewal of the moon, establish for yourselves the new month - or even if you do not see it, since it is appropriate for [the moon] to appear according to the accepted calculation. Likewise, this commandment includes the commandment of intercalation, since the basis of the commandment to sanctify the month is for Israel to observe God's holidays at their appointed times. And the commandment of intercalation has the same basis. However, beyond this verse are more Torah passages concerning the commandment of intercalation, and that is what is written (Exodus 13:10), "You shall observe this commandment in its proper time"; and likewise, "Observe the month of Spring" (Deuteronomy 16:1).

The content of the commandment is that two Jews fit to testify come before the court and testify before them that they saw the renewed moon. [The judges then] establish the new month on the basis of their [testimony] and say, "Today has been sanctified." The reason why this commandment applies only with ordained judges is because it came with the explanation [of the verse], "This month shall be for you" - great [in Torah] and ordained, like you (Rosh Hashanah 22a), since it was said to Moshe and Aharon. This matter was further expounded, as it is juxtaposed to "speak to the entire congregation of the children of Israel," meaning that they who sanctify the month must have the assent of all Israel, meaning the wisest men of Israel, such as the Great Court (the Sanhedrin). And so [too] any great sage in Israel who was ordained in the Land of Israel. And since ordination is no longer practiced, as is known, [the sage] has permission to perform this commandment even outside the Land of Israel, so long as there is no one of his stature in the Land of Israel. And so we find that Chananiah son of Rabbi Yehoshua's brother, and Akiva ben Yosef did so in this regard (Berakhot 23a). But without these conditions, no one of Israel is permitted to establish the months or intercalate. And perhaps you will ask, "If so, how do we conduct ourselves today, when we lack ordained sages?" Know that so have we received it; since Rabbi Hillel the Prince, son of Rabbi Yehudah the Prince, who was the greatest of his generation and ordained in the Land of Israel - he was the sage who fixed for us the calculation of intercalation, sanctified future months, and intercalated future [months into] years until the coming of Eliyahu, and it is upon this that we rely today. That which we have said all follows the opinion of Rambam. However, Ramban (in the Sefer HaMitzvot LaRambam, Mitzvot Ase 153), considers the sanctification of the month as one commandment and intercalation a separate commandment. His proofs (for dividing these into two separate commandments are) in his Book of Commandments. And so [too] the Ba'al Halakhot (considers these separate commandments). And the verse that teaches about the commandment of intercalation, that is, that we calculate the seasons in order to make the holidays in their (proper,) established times, is "You shall observe this commandment in its proper time," (Exodus 13:10) and so, "Observe the month of Spring," (Deuteronomy 16:1) as we have written.

The laws of the commandment - for example, interrogating [those who offer] testimony concerning the new month; instilling fear in the witnesses on occasion; the law concerning circumstances under which the Shabbat may be desecrated for this testimony; for what [considerations] we do or do not intercalate; which month they would intercalate, i.e. Adar, as they, may their memory be blessed, expounded, "'You shall observe this commandment in its proper time' (Exodus 13:10), this teaches that we only intercalate at the time near the holiday"; and they, may their memory be blessed, further expounded on this verse, "From where do we know that we only intercalate the month during daytime? [We learn this from the] verse, [which] states, 'from year to year' (yamim yemima, literally from day to day) (Sanhedrin 10b); and they, may their memory be blessed, further expounded, "'For the months of the year' (Exodus 12:2) - it is months you calculate towards the year, not days" (Megillah 5a); furthermore did they say on this matter, "'A month of days' (Numbers 11:20), it is days you calculate towards the month, not hours" (Megillah 5a); and the rest of its details - are [all] elucidated in Tractate Rosh Hashanah, and in the first chapter of Sanhedrin, and similarly in Berakhot (see Mishneh Torah, Laws of Sanctification of the New Month 1).