Why the Diaspora Keeps an Extra Day at All

The extra festival day that lets Simchat Torah separate from Shemini Atzeret outside Israel exists because of a specific ancient practical problem: in the Temple period the new month was fixed by witnesses testifying to the sighting of the new moon in Jerusalem, and messengers then carried word of the correct date outward, but communities too far away to receive the messengers in time before a festival kept two consecutive days to cover the uncertainty about which day was actually correct, a precaution that became fixed diaspora practice, Yom Tov Sheni shel Galuyot, even after a fixed mathematical calendar replaced witness testimony in the fourth century and removed the original uncertainty entirely.

Most Jewish legal authorities held that diaspora communities should keep the extra day going forward regardless, out of respect for the established custom of the ancestors rather than because any uncertainty still existed, and this same inherited second day is what gives diaspora Orthodox and Conservative communities a spare calendar day on which Simchat Torah can be kept separately from Shemini Atzeret, a distinction that simply has no room to exist in Israel and in Reform practice, both of which keep only the single original day the ancient witnesses would have recognised.