Rosh Hashana 1:4-5

Rosh Hashana 1:4

Witnesses who saw the moon were permitted to violate Shabbos for two months: for Nisan and Tishrei. In those months, messengers were sent to Syria to fix the dates of the holidays. When the Temple stood, the witnesses could violate Shabbos in any month in order to fix the time of the Rosh Chodesh sacrifices.

Rosh Hashana 1:5

The witnesses violate Shabbos to testify about the moon whether it was clearly visible or not; Rabbi Yosi says that they do not violate Shabbos if the moon was clearly visible (because then it must have been seen in Jerusalem).