The time of Passover and the Seder reflects how the Egyptian firstborns were killed during the death of the firstborn plague in Egypt that one night. It was innapropriate to be happy and say the whole entire Hallel at once during this horrific time. We say the whole entire Hallel during the Seder since Passover only involved only a little bit of redemption of the Jewish people and the destruction of Egypt.


haggadah Section: Hallel