The Misva of Kiddush Ha’hodesh requires the leading Sages to declare new months on either the 30th or 31st day of the previous month, based upon the testimony of two witnesses who saw the new moon. The source of this Misva is G-d’s command to Moshe and Aharon in Egypt before the Exodus, “Ha’hodesh Ha’zeh Lachem Rosh Hodashim” – “This month is for you the first of the months” (Shemot 12:2). As the Sefer Ha’hinuch explains, this verse is understood to mean that when the new moon is sighted, the new month is to be declared. Bet Din interrogates the witnesses, and then proclaims, “Mekudash” – that this day is Rosh Hodesh, the first day of the new month.
The Sefer Ha’hinuch clarifies that this Misva also includes the obligation of “Ibur Shana” – the declaration of a leap year when it becomes necessary to adjust the calendar. As opposed to the solar calendar, which is based upon Earth’s 365-day revolution around the sun, the Jewish calendar follows a lunar-based system, with months being determined based on the moon’s revolution around Earth. However, twelve revolutions of the moon take only 354 days – 11 days fewer than Earth’s revolution around the sun, which means that the Jewish year is 11 days shorter than the solar year. The Torah requires ensuring that Pesach is observed in the springtime (“Shamor Et Hodesh Ha’abib Ve’asita Pesach” – Debarim 16:1), and Sukkot is called “Hag Ha’asif” – the festival of the completion of the harvest (Shemot 34:22) and must be celebrated at the time of the gathering of the harvested produce (Debarim 16:13). Therefore, it becomes necessary to occasionally adjust the Jewish calendar to keep it in sync with the solar calendar. This is done by the addition of an entire month – a second Adar – every two or three years. (The Jewish calendar differs in this respect from the system used by the Moslems, which is based solely on the lunar cycle, and which results in Ramadan, for example, falling at all different times of the year.) This obligation of “Ibur Shana,” to add a month to ensure that the holidays fall in their appropriate seasons, is included in the obligation of Kiddush Ha’hodesh.
As mentioned, the command of “Ha’hodesh Ha’zeh Lachem” was directed to Moshe and Aharon, the nation’s spiritual leaders at that time. The Rabbis deduced on this basis that the Misva of Kiddush Ha’hodesh is charged upon the leaders of the generation, the foremost scholars who have received Semicha – formal ordination from a Rabbi who himself received ordination and is part of the chain dating back to Moshe Rabbenu. A Rabbi can perform this Misva only in Eretz Yisrael. However, if he received Semicha in Eretz Yisrael and then moved to the Diaspora, he can nevertheless perform this Misva if he is recognized as the greatest Rabbi in the world in his time. The Sefer Ha’hinuch writes that a qualified Rabbi who, for whatever reason, refrains from involving himself in Kiddush Ha’hodesh is liable to severe punishment, because he causes the holidays to be observed in the wrong time.
Today, we no longer have “Semuchim” – Rabbis with formal Semicha, because the chain of Semicha was broken many centuries ago. The Gemara explains that Hillel, a grandson of Rabbi Yehuda Ha’nasi, established the Jewish calendar for centuries into the future, and he effectively “declared” all the new months henceforth. The Sefer Ha’hinuch writes that all the new months and leap years were arranged by Hillel’s system until the time when Eliyahu Ha’nabi will arrive. Once Mashiah comes, we will return to the original system of witnesses testifying to the sighting of the new moon, and the declaration of new months based on their testimony.
In the Book of Vayikra (23:4), the Torah says about the holidays, “Tikre’u Otam Be’mo’adam” – “you shall declare them at their times.” The Gemara notes that the word “Otam” (“them”) in this verse is written without the letter “Vav,” such that it may be read as “Atem” – “you.” This word, the Gemara establishes, indicates that the Rabbis’ declaration of the new month is binding even if it was mistaken, and even if they intentionally declared Rosh Hodesh on the wrong day. Remarkably, G-d empowered the Rabbis to such an extent that the days on which the holidays fall according to their pronouncements are considered sacred – even if they made a mistake, and even if they made an intentional mistake.
The question arises as to whether this principle applies also to the declaration of leap years. If the Rabbis declared an extra month when this was not warranted, is their declaration binding?
The Noda Bi’yehuda (Rav Yehezkel Landau of Prague, 1713-1793), in his Talmud commentary (called “Selah” – “Siyun Le’nefesh Haya”), writes that this principle does not apply to the declaration of leap years. He notes the story told of King Hizkiyahu, who once declared a leap year on the 30th of Adar, after that day had already been declared the first of Nissan, such that it was already too late to add a second Adar. The Sages of his time disapproved of this declaration, and Hizkiyahu regretted his mistake and begged G-d for forgiveness. This appears to demonstrate that mistaken declarations of a leap year are not binding.
However, the Minhat Hinuch cites a passage from the Midrash (Torat Kohanim) stating explicitly that if the Rabbis pronounced a leap year when a leap year was not required, their declaration is binding – in direct contradistinction to the Noda Bi’yehuda’s conclusion. As for the story of Hizkiyahu, the Minhat Hinuch writes that this situation involved not a mistaken declaration of a leap year, but a declaration made after the time when this is allowed. The Minhat Hinuch explains that although the mistaken declarations of new months and leap years are binding, this applies only if the declarations followed the basic framework of the calendar system. For example, if the Rabbis declared Rosh Hodesh on the 15th of the month, or even on the 28th of the month, their declaration is entirely meaningless. It is only if they declared the 29th day Rosh Hodesh instead of the 30th, or the 30th instead of the 29th, that their mistaken pronouncement is binding. Likewise, in the case of the declaration of a leap year, the declaration is binding only if it was done at the appropriate time. If it was unnecessary, it is nevertheless binding, because the process was done according to the accepted system. Hizkiyahu declared a leap year after the final time when this is allowed, once the month of Nissan was declared to have begun, and thus his pronouncement was not valid.