Why is Passover observed for seven days in Israel but for eight days outside of Israel?
Exodus 12:14 commands Jews to observe Passover for seven days, stating: "This is a day you are to commemorate; for the generations to come you shall celebrate it... for seven days you are to eat bread made without yeast." An extra day was added to the observance of Passover following the exile from ancient Israel in 70 C.E. This exile occurred after the destruction of the Second Temple and is when a large number of Jews began living in countries all over the world.
Why was an extra day necessary? The answer has to do with the way the ancient calendar worked. The Jewish calendar is lunar based instead of solar based like the secular calendar. Ancient Jews didn’t use nifty wall calendars to track the dates like we do today – rather, each month began when witnesses spotted the New Moon in the sky. There had to be at least two male witnesses, who would testify about what they had seen to judges based in Jerusalem. Once the judges verified that the men had seen the correct phase of the moon, news about the start of the month was sent from Jerusalem to places far and wide. Because it could take a while for news to reach territories outside of Israel and because mistakes could possibly be made along the way, an extra day was added to the observance of Passover in order to prevent people from accidentally ending the holiday early.
Many modern Jews in the Diaspora observe Passover for the full eight days, although Reform and Conservative Jews will sometimes observe only seven days as if they were in Israel.