1,267. Observing Two Days of Rosh Hashana Even in Israel

Hilchos Kiddush HaChodesh 5:7

Even when the calendar was set based on seeing the moon, most residents of Israel observed Rosh Hashana for two days out of doubt. They wouldn’t know the day the court had established as the new month because messengers weren’t sent on yom tov.

Hilchos Kiddush HaChodesh 5:8

Even in Jerusalem, where the court sat, Rosh Hashana was often observed for two days because if witnesses did not arrive on the 30th day, the day on which they expected witnesses was treated as yom tov, as was the next day. Since Rosh Hashana was observed for two days even when the moon was sanctified based on witnessing the moon, the Sages decreed that even residents of Israel should observe Rosh Hashana for two days nowadays, when we base the holidays on the fixed calendar. Therefore, even the second day of Rosh Hashana is a rabbinic enactment nowadays.