Q. Why is the door opened after the meal on Seder night?
A. Most people link the custom with the prophet Elijah and believe we open the door as a welcome to the prophet, coming to announce the Messiah. But the circumstances that gave rise to the opening of the door were more prosaic.
In the Middle Ages several families would sometimes gather in the home of one of them to hear the Haggadah. When the time came for the meal, they would go home to eat. In some cases a more learned person would go from one house to another to read the Haggadah for those who could not do so for themselves. He would then proceed to his own house to eat his Pesach meal.
Since Pesach was often the occasion of antisemitic attacks, it was not always easy for people to return for the second section of the Seder, and the door would be opened in the hope that God would protect them and they would get back safely.
No wonder the opening of the door was accompanied with the Biblical verse, “Pour out Your anger upon the heathen who know You not” (Psalm 79:6).