1,269. When There Is No Received Tradition

Hilchos Kiddush HaChodesh 5:11

The factors determining whether a place observes one day of yom tov or two are as follows: if the distance between Jerusalem and a given location is a journey of more than ten days, the residents should observe two days of yom tov, as they did before the fixed calendar. This is because the Tishrei messengers couldn’t be expected to reach places outside of a ten-day journey from Jerusalem. If a place was a journey of ten days or less from Jerusalem so that the messengers could have reached them, then we see whether the place was a part of Israel that was inhabited by Jews during the second conquest when the calendar was established based upon seeing the moon. Such places - including Usha, Shefaram, Luz, Yavneh, Nov, Tiberias, et al. – observe just one day of yom tov. If a place is part of Syria (i.e., lands annexed by Israel in the time of King David), they should follow the practice of their ancestors. Such lands include Tyre, Damascus, Ashkelon, etc., some of which observed one day and some of which observed two.

Hilchos Kiddush HaChodesh 5:12

If a place is within a journey of ten days or less from Jerusalem, is part of Syria or the diaspora, and has no received tradition regarding yom tov, they should observe two days like most of the world does. The same is true of a city that was established in the wilderness of Israel or a city that first became populated by Jews in the modern day. Nowadays, observing a second day of yom tov is always of rabbinic origin; this is so even of the second day of Rosh Hashana, which is currently observed by Jews everywhere.