There appears to be a conflict between the Synoptic Gospels (Matthew, Mark and Luke) and the Gospel of John. See Baruch M. Bokser, “Was the Last Supper a Passover Seder?” BR, Summer 1987; Jonathan Klawans, “Was Jesus’ Last Supper a Seder?” BR, October 2001; and the dated but still helpful study by Joachim Jeremias, The Eucharistic Words of Jesus (London: SCM, 1966).