1,265. The Establishment of the Fixed Calendar

Hilchos Kiddush HaChodesh 5:3

The Jewish people started using the fixed calendar at the end of the Talmudic era, when Israel was in ruins and the established court no longer remained there. In the time of the Mishna and Gemara until the time of Abaye and Rava, the people still relied on the calendar being set in Israel.

Hilchos Kiddush HaChodesh 5:4

In the time of the Sanhedrin, when the calendar was based upon seeing the moon, the residents of Israel wherever the Tishrei messengers would reach observed the holidays for one day, while residents of distant places where the Tishrei messengers did not reach would observe two days out of doubt because they didn’t know the day that the residents of Israel had set as Rosh Chodesh.