For those outside of Israel the requirement is for two days of yom tov (and sometimes Rosh Chodesh), originally due to uncertainty and now due to tradition. But we don't do this for all holidays even ...
There is an article on Wikipedia about Passover that gives a very non-traditional (if not anti-traditional) explanation - rather, a series of explanations - for the origin of Y"T Sheni Shel Galuyoth. ...
Why do we continue to keep two days of Yom Tov outside of Israel? The reason for two days was revelant during the temple era as we needed two witnesses to see the new moon. But today, with no Temple ...