Yom tov sheni shel galuyot (Hebrew: יום טוב שני של גלויות), also called in short yom tov sheni, means "the second festival day in the Diaspora", and is an important concept in halakha (Jewish law). The concept refers to the observance of an extra day of Jewish holidays outside of the land of Israel.
The need for a second festival day arises from problems encountered by Jews living in the Diaspora following the Babylonian exile. The Jewish calendar, is a lunar system with months of 29 or 30 days. In Temple times, the length of the month depended on witnesses who had seen the new moon coming to the Temple in Jerusalem. Following confirmation of their evidence, a new Jewish month would be proclaimed. News of this proclamation was subsequently sent out to all Jewish communities. If no witnesses arrived, the new month was proclaimed the following day. Those communities who didn't receive word of the precise date of the beginning of the new month by the time of a festival, would keep the festival for two days, to account for the eventuality the new month wasn't proclaimed only the following day.