I have never understood this: why do some synagogues (like Temple Emanuel) celebrate two days of Rosh Hashanah, while others only celebrate one?
The origins of Rosh Hashanah’s two days is connected to the 2 day celebrations at the beginning of the three Pilgrimage Festivals (Sukkot, Pesach, Shavuot) and goes back to the time of the Mishneh (around 200 CE).
The beginning of the new month was announced by the Rabbinic Court in Jerusalem, who would hear eyewitness testimony, verify it through questions, and determine the new month, and new moon had begun. Beacon res, seen from a long distance was lit, and watchers on faraway hills set their beacon res as soon as they saw them, continuing the relay “until one could behold the whole of the Diaspora before him like a mass of re” (Mishnah Rosh Hashanah 2:4). Later, messengers were used, and it could take a long time for them (and the beacon res) to reach the whole of the diaspora to announce the beginning of the new month.
Due to the time it took for the new month to be announced, those outside of Jerusalem could not be sure when the festivals began, so they celebrated two days to ensure they celebrated at the correct time. Rosh Hashanah has an added difficulty. Since Rosh Hashanah is the rst day of the month, no messengers could be sent because the Holy Day began, and they couldn’t work, so the announcement would have to wait one day. The rabbis determined that if two days were celebrated, even in Jerusalem, the chances were that one of those days would indeed be the holiday.
Since in modern times we are able to determine, with certainty, when the month begins, the Reform movement believed that there was no need to continue to celebrate two days at the beginning of the festivals, so Reform Jews only celebrate one day. There are still some congregations (like ours) that continue two days of Rosh Hashanah, and Israel also celebrates two days, to honor the ruling of the Rabbis who suggested two days of celebration to be sure it is done on the correct day.