Rosh Hashanah is the one festival about which Israelis cannot say that they have it easier than the diaspora with only one day of Yomtov. Rosh Hashanah here is two days, like all the Yomtovim in the diaspora.
(OK, there’s Yom Kippur, but two days of Yom Kippur was always a non-starter due to the potential dangers of fasting for 49 hours.)
According to the Torah, Rosh Hashanah is one day, the first of the seventh month. How did it become two? Until the year 358, the Great Sanhedrin (Supreme Court) declared the start of every month when two witnesses testified to seeing the new moon.
A torch system and, later, messengers spread the word. Far-flung communities could not be sure that they would know about the start of the month on time and therefore observed two days of Yomtov to avoid mistakes.
As Rosh Hashanah falls on the first day of the month, the messengers announcing the new month could arrive on Rosh Hashanah itself. Even those living in Jerusalem might not know until the day had begun that it was Rosh Hashanah.
To avoid such confusion, two days of Rosh Hashanah were instituted for Israel.
Rosh Hashanah is known as Yoma Arichta, one long day. Whatever the origins, two days are still barely long enough to review the previo