Note 1 at Lu 22:7: Matthew, Mark, and John all made a point of saying that these events took place on the first day of the feast of unleavened bread. Luke added the additional statement that this was the day when the Passover lamb must be killed. Matthew's and Mark's accounts go on to say that it was that evening when Jesus ate the Passover meal with His disciples (Mt 26:20 and Mr 14:17).

This looks like it contradicts Joh 18:28 where it speaks of the Jews preparing to eat the Passover after they delivered Jesus to Pilate the next morning. Joh 19:14 further complicates things by saying that the sixth hour on the day of Jesus' crucifixion was the preparation of the Passover, implying that it was the next day when the Passover was actually eaten.

All of this can be harmonized by remembering that the Jewish day did not begin at midnight, as we are accustomed to. The Jewish day ran from sundown of one day to sundown of the next. Therefore, when Mt 26:17, Mr 14:12, and Lu 22:7 speak of the first day of unleavened bread arriving and so the Passover must be killed, they are speaking of the evening before the actual death of the Passover lamb.

Jesus and His disciples ate the Passover meal (see note 2 at Lu 22:15) in the evening of the thirteenth of the month of Abib, or Nisan, one day early. This was because when everyone else was eating of the Passover lamb the next evening, Jesus would have already offered Himself as our Passover Lamb, and He would be in the grave. That same night Jesus was arrested in the garden (Mt 26:57, Mr 14:46, Lu 22:54, and Joh 18:12) and taken before the high priests the next morning, which was still the same Jewish day. This was the day that Luke stated here "when the passover must be killed."

Joh 19:14 calls this day the preparation of the Passover because the actual Passover meal was not eaten until that evening, and that, according to Jewish reckoning, was the fourteenth day of the month of Abib (Nisan - Ex 12:18, 13:4; Ne 2:1; and Es 3:7).

In the Old Testament, allowances were made for those who could not eat the Passover meal on the fourteenth day of the first month to eat this meal in the second month on the fourteenth day (Nu 9:2-14). Therefore, it is certainly understandable that our Lord, who was our Passover Lamb (1Co 5:7), could eat the Passover meal with His disciples one day early.