For two months, the witnesses may desecrate the Sabbath: for Nissan, and for Tishrei, for on them the messengers go forth to Syria, and on them they fix the holy days. And when the Temple was in existence, they desecrated even for all of them because of the determination [of the time] of the sacrifice.

Whether it [the new moon] was clearly visi-ble, [or] whether it was not clearly visible, they could desecrate the Sabbath because of it.  R’ Yose says: If it was clearly visible, they could not desecrate the Sabbath because of it.