Why is Passover celebrated for 8 days?

The dates and length of Jewish festivals are decided according to the complex Hebrew calendar, which involves both lunar and solar measurements taken in Israel. As in the past getting the relevant information from Israel to the farthest reaches of the world would take far longer than today, Jews in other nations added an extra day to the festivals and holidays to ensure they were celebrating the event on the correct day and, over time, this became a tradition.

Today, even though we have very fast worldwide communication networks which would be more than capable of allowing Jews in any country to get the information, festivals are often celebrated for one day more in the Diaspora (Jewish communities outside Israel) than they are in Israel - Passover is one example of this.