The 153rd mitzvah is that G‑d (exalted be He) commanded us to calculate the months and years.1 This is the mitzvah of Kiddush HaChodesh (Sanctifying the Moon).

The source of this commandment is G‑d's statement2 (exalted be He), "[And G‑d said to Moshe and Aharon in the land of Egypt,] 'this month [Nissan] shall be the head month to you.' "

In their explanation of this mitzvah, the Sages said,3 "This testimony is given lochem ["you", plural]."4 The meaning of this statement: This mitzvah is not incumbent on every individual as is Shabbos, for example, where every single individual counts six days and rests on the seventh. In our case, it would mean that every individual who sees the appearance of the new moon would consider that day Rosh Chodesh [the first of the month]; Or that an individual could use the Torah-approved calculations to himself determine Rosh Chodesh; Or that he could himself estimate that the produce would not yet ripen [by Pesach]5, or consider any of the other factors6 which are used in determining [the calendar] — and then [himself declare a leap year and] add a month!

However, this mitzvah can only be performed by the Bais Din Hagadol, and only in Eretz Yisroel7. Therefore, since today there is no Bais Din HaGadol, we no longer determine the months on the basis of testimony, just as we no longer bring sacrifices because there is no Holy Temple.

The group of heretics known here in the East8 as Karaites have erred in this principle.9 Not even all of the Rabbis have grasped it, and as a result, grope around with them together in deep darkness.10

One must understand that the calculations which we use today to know11 when Rosh Chodesh and the holidays occur, may only be done in Eretz Yisroel. Only in cases of dire need, when there are no Sages in Eretz Yisroel, and when the Bais Din outside Eretz Yisroel was previously ordained in Eretz Yisroel, is it permissible to declare a leap year or determine Rosh Chodesh outside Eretz Yisroel — as Rabbi Akiva did, as explained in the Gemara.12 This is a very extreme measure, and it is well known that in the majority of cases, it was done only in Eretz Yisroel. They [i.e. the Sages in Eretz Yisroel] are the ones to establish the months and declare a leap year, when they gather together and use the accepted methods.

There is a very important principle upon which the Torah's perspective13 on this subject is based, which is only understood and fully realized by those who delve deeply into the Torah, as follows. This that we outside Eretz Yisroel use our system to make calculations and we declare that "this day is the first of the month," and "this day is a holiday," does not in any way mean that we are making this day based on our calculations. Rather, it is because the Bais Din in Eretz Yisroel has already established that the day is a holiday or Rosh Chodesh. The day becomes a holiday or Rosh Chodesh upon their declaration, "Today is Rosh Chodesh, or "Today is a holiday"; regardless of whether they based their actions on calculations or testimony.14

This [that the Bais Din HaGadol in Eretz Yisroel has absolute authority] is known to us through the verse15, "[Speak to the Israelites and tell them, 'These are the holidays] that you shall designate.' " Our Sages explain16, "These are the only holidays." The meaning of this statement, as passed down in the Oral Tradition: whatever they [i.e. the Bais Din] designate as holidays are considered holidays, even if they made an error, were forced [into making a declaration], or misled.

The calculations which we make today are only to know which day they established in Eretz Yisroel, since they use the exact same system to make calculations and to determine the day – not testimony. Therefore, we are really basing ourselves on their determination, rather than our own calculations, which are only used to reveal [what they already determined previously]. One must clearly understand this.

I will give some additional explanation: let us assume, for example, that there would be no Jewish inhabitants in Eretz Yisroel (G‑d forbid such a thing, since He has already promised that he will never completely wipe out or uproot the Jewish nation17); that there would be no Bais Din there, nor a Bais Din outside Eretz Yisroel which had been ordained in Eretz Yisroel. In such a case, our calculations would be totally futile,18 since we, who dwell outside Eretz Yisroel, may not make the calculations, nor declare leap years nor establish the months without the conditions mentioned above,19 since, "For from Zion shall go forth the Torah, and the word of the L‑rd from Jerusalem."20 A person who fully understands the words of the Talmud in this subject will, upon meditation, undoubtedly agree with the abovementioned.

The Torah contains brief references to the basic principles which are relied upon to know when Rosh Chodesh and the leap years occur. Among them: "This law must therefore be kept at its designated time (l'moadah)."21 Our Sages said,22 "This teaches you that one may add on to the leap year only close to the holiday" [moed, i.e. Pesach23].

They also said,24 "From which verse do we derive that only during the daytime may we officially add on to the month or officially declare Rosh Chodesh? From the verse,25 'miyamim yamimah.' "26

[So too,] G‑d's statement27 (exalted be He), "[This month shall be the head month to you; the first month] of the months of the year." On this our Sages said,28 "The year must be composed of months, not of [odd] days", meaning that when adding on to the year, a complete month must be added.29

There is also a verse,30 "a month of days." Our Sages commented,31 "the month must be composed of days, not of [odd] hours". [So too] the verse,32 "safeguard the month of Aviv,"33 which implies that in calculating the year we must take into account the seasons. Therefore, they shall be years [based not only on the moon but also] based on the sun.

All the details of this mitzvah have been completely explained in the first chapter of Sanhedrin,34 in tractate Rosh Hashanah,35 and in Berachos.