As a child, my family always used to have a second, smaller seder on the second night of Passover. When and why did this custom start?
The practice of a second seder developed as a result of the dispersion. The dating of the calendar was based upon the time zone that Israel was in. When the Jews were scattered throughout the world, it was not always clear exactly when the first night of Passover fell in conjunction with the time in Israel as over against the time elsewhere in the world. To make sure that the Jews observed the seder on the correct day, it became a tradition to observe it for two days straight, to make sure it fell on the right day. That is also true about the festivals. Most Jewish festivals are observed one day longer than they would be within the land of Israel.