It can be difficult to make sense of the timeline for Jesus death when considering the data presented in the Synoptics vs what is presented in the Gospel of John. What seems clear is that in the synoptics the lambs are slaughtered on the Day of Preparation, and the last Supper is the Passover meal (e.g. Mk 14:12-16). For John it is important to stress the theological point that Jesus is the lamb of God. So for John Jesus is crucified at the same time the Passover lambs would normally be slaughtered, and the crucifixion is already done and over with by the time he would supposedly have consumed the Passover meal as per the synoptics (Jn 19:14-15,31).
To fuller understanding of this timeline, a helpful table was created by a man named Felix Just which is available here. This table compares Mark's passion presentation with that of John so that the distinctions are apparent.