It was on Passover that the early church celebrated the redemption from sin that we now call Easter. It wasn't until the fourth century that the feast was moved to the first Sunday after the full moon after the vernal (sometimes called paschal) equinox. Since different traditions use different calendars, the Western and Eastern Christian churches often observe Easter on different dates. But the symbolism of going from darkness to light, from slavery to freedom, from death to life remains consistent. Look at the beautiful opening words of the Anglican Easter vigil: "On this most holy night, in which our Lord Jesus Christ passed over from death to life, the Church invites her members, dispersed throughout the world, to gather in vigil and prayer. For this is the Passover of the Lord, in which through word and sacrament we share in his victory over death."
By saying all of this I don't want you to think that I'm in any way trying to "Christianize" the Seder. Not at all. Even when Seders take place in churches, Catholic bishops discourage any attempt to "baptize" passover, advising, "When Christians celebrate this sacred feast among themselves, the rites of the Haggadah for the Seder should be respected in all their integrity. The Seder . . . should be celebrated in a dignified manner and with sensitivity to those to whom the Seder truly belongs." And the Seder belongs to the Jewish people. I am just grateful that I am able to share in this night that is truly different from all others.