In Israel the three Festivals are 7 days long. The Festivals, Sukkot, Passover and Shavuot, were the times when the ancient Israelites traveled to the Temple in Jerusalem to make sacrifices, as commanded in the Torah (five books of Moses).
In Israel today the Festivals are still observed for 7 days. When the ancient Temple was destroyed and the Jews were driven from Israel Jews were dispersed throughout the Diaspora. Jews living outside of Israel were not sure when a Festival or holiday began. Ancient communications included someone building a bonfire on a hill in Israel and a distant community seeing the fire and lighting another fire to pass on the word. It was faster to communicate the date this way than to send messengers, although messages and messegers were also used. Still it meant that Jews in the Diaspora were worried that they may not be celebrating on the correct date. To hedge their bets they added a day. Thus outside of Israel it became the custom that festivals were observed for 8 days.
In modernity we know exactly what day it is. Based on this, the Reform movement eliminated the 8th day and celebrates the Festivals on the same days as does Israel. However, the Conservative and Orthodox movements retained the tradition of eight days for each Festival. For all Jews Passover began this year on Monday, March 25 at sundown. But for the Reform movement it ended yesterday evening, Monday, April 1. For Conservative and Orthodox Jews, Passover 2013 (or 5773 on the Jewish calendar) ends Tuesday night, April 2.