1,266. Observing Two Days of Yom Tov Outside of Israel

Hilchos Kiddush HaChodesh 5:5

Nowadays, in the absence of a Sanhedrin, since the court of Israel sets the months according to the fixed calendar, it would be reasonable for Jews worldwide to observe the holidays for just one day alone because the residents of distant lands and those of Israel rely on the same calendar and set the holidays the same way. Nevertheless, the Sages decreed that residents of distant lands retain the practice of their ancestors.

Hilchos Kiddush HaChodesh 5:6

Therefore, places that the Tishrei messengers would not reach when the messengers were sent out observe two days even now, just as they did when the people of Israel based the calendar on witnessing the moon. Residents of Israel continue to observe the holidays for one day because they never observed two days. Our current practice to observe a second day outside of Israel is therefore a rabbinic enactment.