1,251. Where Messengers Did Not Arrive

Hilchos Kiddush HaChodesh 3:11

Wherever the messengers arrived, holidays were observed for one day as described in the Torah. In the faraway places that the messengers did not reach, the holidays were observed for two days out of doubt because they didn’t know which day the court might have designated Rosh Chodesh.

Hilchos Kiddush HaChodesh 3:12

There are places where the messengers for Nisan would arrive in time but those for Tishrei would not arrive. It would have seemed reasonable for the residents of such places to observe one day of yom tov for Passover – since the messengers reached them with news of when Rosh Chodesh was – but to observe two days of yom tov for Succos since the messengers did not reach them. However, so as not to make a difference between the two holidays, the Sages ruled that two days of yom tov were to be observed anywhere that was not reached by the messengers of Tishrei. This is so even for the holiday of Shavuos (which is counted from the omer offering and is not dependent on Rosh Chodesh).