Rosh Hashanah, Chapter Two, Mishnah 7. In the previous section we learned about the process by which witnesses are accepted in beis din; now the Mishnah teaches us how the act of sanctifying the month itself actually takes place.
The text of the Mishnah:
"Rosh beis din omer mekudash, v'chol ha'am onin acharav mekudash mekudash" - the head of the court declares that the month is sanctified, and all the people present there answer after him: "Sanctified, sanctified."
"Bein nir'ah bizmano bein shelo nir'ah bizmano mekadshin oso":
There are two possible days on which the moon may be sighted, and "seen at its proper time" means that it was seen on the first of them - the thirtieth day of the previous month, that is, that the witnesses testified that they saw it on the thirtieth day. According to the first opinion, whether it was seen at its proper time or whether it was not seen at its proper time but only on the following day, in both cases the act of sanctification is performed: the head of the court says "Sanctified," and all the people answer after him "Sanctified, sanctified."
"Rabbi Elazar b'Rabbi Tzadok omer":
Rabbi Elazar b'Rabbi Tzadok disagrees and holds that if it was not seen at its proper time, meaning on the first day, "ein mekadshin oso" - the act of sanctification is not performed at all. His reason: "shekvar kidshuhu Shamayim" - once the month has been sanctified in Heaven, it is sanctified on its own as the default, and there is no need for the court to become involved. This is something that happens by itself, and therefore there is no need to actually carry out the procedure of declaring "Sanctified, sanctified."
In summary: in this Mishnah we learned the order of the act of sanctifying the month - the declaration of the head of the court and the people's response after him - and the dispute among the Tannaim: according to the first opinion the month is sanctified in every case, whether it was seen at its proper time on the thirtieth day or whether it was not seen at its proper time; while according to Rabbi Elazar b'Rabbi Tzadok, when it was not seen at its proper time it is not sanctified, since Heaven has already sanctified it.