Q: 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?
A: 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.
Q: How did the early Church, Jew and Gentile, celebrate the resurrection of Jesus? Did they continue in their week-long Passover, or abandon it like the Church of today?
A: The Jewish believers continued (as many still do today) observing the whole Passover week (Passover day plus seven days of Unleavened Bread). However, the Gentile churches largely celebrated only His death and resurrection and did so primarily at the time of Passover, though they limited it to a shorter period than did Jewish believers.
As far as modern practice is concerned, we are to observe and remember His death and His resurrection. When Jesus said, “do this in remembrance of Me,” He was speaking only of the bread and the cup, not the entire festival meal. [The Passover observance was mandatory under Mosaic Law, but the Mosaic Law was rendered inoperative as the rule of life when Messiah died.] Therefore, while believers today (Jew or Gentile) are certainly permitted to observe the whole Feast of Passover, they are only commanded to observe the bread and the cup.