Many people assume that Jesus’ Last Supper was a Seder, a ritual meal held in celebration of the Jewish holiday of Passover. And indeed, according to the Gospel of Mark 14:12, Jesus prepared for the Last Supper on the “first day of Unleavened Bread, when they sacrificed the Passover lamb.” If Jesus and his disciples gathered together to eat soon after the Passover lamb was sacrificed, what else could they possibly have eaten if not the Passover meal? And if they ate the Passover sacrifice, they must have held a Seder.
Three out of four of the canonical Gospels (Matthew, Mark and Luke) agree that the Last Supper was held only after the Jewish holiday had begun. Moreover, one of the best known and painstakingly detailed studies of the Last Supper—Joachim Jeremias’s book The Eucharistic Words of Jesus—lists no fewer than 14 distinct parallels between the Last Supper tradition and the Passover Seder...more...
It's not so simple to answer the question of whether the Last Supper described in the New Testament was a Seder. Jonathan Klawans does a good job of summarizing the state of the question in, "Was Jesus’ Last Supper a Seder?" in the current BAR.