Yom Tov Sheni

Hi,

Today we begin a new topic: The special status of the extra day of Yom Tov which is added outside of Israel [Yom Tov Sheni shel Galuyot].

The lunar cycle takes 29.5 days. Therefore, some months will be 29 days and some 30, depending on the time of day on Rosh Chodesh, when the old cycle ends and the new cycle begins.

In the era when the start of the lunar month was fixed by a central court based on the testimony of witnesses as well as social needs, Jewish communities who would not receive news of the central court's decision in a timely manner would not know whether the previous month had been 29 or 30 days, and this would affect celebration of holidays. Therefore, those communities observed an extra day [Yom Tov Sheni shel Galuyot] for Succot/Shmini Atzeret, Pesach and Shavuot, to be certain that they were observing the correct day.

For the past 1500 or so years, Jews have used a fixed, pre-calculated calendar, instead of testimony and courts. Nonetheless, Jews who live in communities that historically would have needed an extra day continue that custom. The gemara notes a concern that, as occurred for some communities during World War II, Jewish communities might at some point be unaware of the correct day for Rosh Chodesh.

(Talmud, Beitzah 4b; Shulchan Aruch Orach Chaim 496:1)

Have a great day,
Mordechai