If one reads the text simply with Jewish concepts in mind, the subsequent account creates confusion once more, since Jesus and the disciples speak of preparations for the Passover meal (Matt. 26:17b, 18b, 19 and parallels). But this passage mentions only the room, and there is no hint of any lamb to be sacrificed at the Temple. The point is instead just this: on the evening of the day in question (Matt. 26:20 and parallels), to the astonishment of his disciples, Jesus held a covenant meal similar to the Passover meal but nevertheless novel..... Surprisingly, however, this nocturnal common meal was so structured that Jesus announced his coming betrayal and, with that in mind, established a new sacrificial covenant (Matt. 26:20-29 and parallels). Here again we must avoid any mechanical association with Jewish concepts. In fact, not one word in the Synoptic accounts suggests that for the Last Supper Jesus had a Passover lamb slain and roasted.... The absence of all mention of a lamb can hardly be dismissed as accidental; for, in view of the many ceremonial details of the account, such carelessness with regard to a major aspect of the Passover festival would be most strange. In fact, the symbolic offering of Jesus' blood at this point (Matt. 26:28 and parallels) appears to exclude the presence of a ritually sacrificed Passover lamb, because this lamb would be eaten without blood and would therefore not lend itself to such symbolism. Apart from all other considerations, the remarkable absence of the Passover lamb can best be explained by assuming that Jesus celebrated the Last Supper during the night of the Day of Preparation and before the day when the Passover lamb was sacrificed. Furthermore, this very vacuum gives a liturgically reasonable form to the establishment of the new Passover sacrifice: Jesus himself can now appear as the Paschal Lamb, as John and Paul represent him (John 1:29; I Cor. 5:7).
from pages 181-182, The New Testament Era