The dates for Passover change because it is based on the phases of the moon. Some Jewish people observe Passover for eight days instead of seven. When Jewish people began to spread around the world, it became more difficult to observe the dates in alignment with those in Israel. As a result, some Jewish people choose to extend the last night of celebration to two nights.
The first and last days of Passover are considered special. Services are held on those days. On the first and second nights, the story of Exodus is retold.Learn more about Passover