Yom tov sheni shel galuyot (Hebrew: יום טוב שני של גלויות), also called in short yom tov sheni (Hebrew: יום טוב שני), means "the second festival day in the Diaspora".
This is a principle in halakha that mandates the observance of an additional day for Jewish holidays outside the Land of Israel.
[1] Yom tov sheni was established as a gezera (rabbinic law) by the rabbis of the Sanhedrin in the Second Temple period, approximately 2,000 years ago, and is observed to this day by Orthodox and Conservative Jews.
The need for a second festival day arises from problems encountered by Jews living in the Diaspora following the Babylonian exile.
Nevertheless, rabbinic authorities decreed that Diaspora communities continue to observe two days of holidays, for two reasons: to preserve their ancestral custom; and out of fear that the non-Jewish authorities might prohibit Torah study and Diaspora Jews would no longer know how to reliably calculate the calendar.
Conservative Judaism uniformly observes two days of Rosh Hashanah as well, as do some Reform congregations.
[10] The opinion of the Chacham Tzvi in his responsa[11] is that a person from abroad who is in the Land of Israel is obligated to observe only one day, and there is room to infer this from the opinion of Rabbi Shneur Zalman of Liadi in his Shulchan Aruch, in its second edition.