In addition to being tied to nature, the Jewish calendar is also connected to Jewish history. It was the custom in the Second Temple period to start the new months in the Jewish communities in Babylonia (present-day Iraq) and elsewhere when the new moon was sighted in Jerusalem. Because of the uncertainty surrounding when the new moon appeared in Jerusalem, it became a Jewish custom outside of Palestine to observe all holidays for two days. Some Jews still do this, and hold a seder on both the first and second night of Passover. Others hold only one seder, on the first night.
The Guide to Passover for Interfaith Families is also available in PDF