See this passage explained in the Matthew 26:17-19 notes, and Mark 14:12-16 notes.

The passover - Πασχα, Luke 22:1, is the name of the festival; το πασχα here is supposed to be the name of that on which they feasted, viz. the sacrificed paschal lamb. But see the notes on Matthew 26 (note), and especially the observations at the end of that chapter, (Matthew 26:75 (note)).

Then came the day of unleavened bread,.... The first of them, the fourteenth day of the month Nisan:

when the passover must be killed; that is, the passover lamb, as the Persic version renders it; and which, according to the law in Exodus 12:6 was to be done between the two evenings; See Gill on Matthew 26:17.

{3} Then came the day of unleavened bread, when the passover {c} must be killed.

(3) Christ teaches his disciples by an obvious miracle that although he is going to be crucified, yet nothing is hidden from him, and therefore that he is going willingly to death.

(c) By the order appointed by the law.

22:7,8 Then came the day of unleavened bread. Josephus calls both the fourteenth and fifteenth of Nisan, the day of unleavened bread. I have followed Neander, Tischendorf, Winer, Alford, Ellicott, Erasmus, Grotius, Calvin and others in the view that the Lord's passover was eaten one day before the Jews ate theirs, and that he died about the time that the paschal lambs were slain for the Jews' passover. See notes on Joh 18:28 19:31.

22:7 Mt 26:17; Mr 14:12.

Lu 22:7-38. Last Passover-Institution of the Supper-Discourse at the Table.

7. the day of unleavened bread-strictly the fifteenth Nisan (part of our March and April) after the paschal lamb was killed; but here, the fourteenth (Thursday). Into the difficult questions raised on this we cannot here enter.

22:7-18 Christ kept the ordinances of the law, particularly that of the passover, to teach us to observe his gospel institutions, and most of all that of the Lord's supper. Those who go upon Christ's word, need not fear disappointment. According to the orders given them, the disciples got all ready for the passover. Jesus bids this passover welcome. He desired it, though he knew his sufferings would follow, because it was in order to his Father's glory and man's redemption. He takes his leave of all passovers, signifying thereby his doing away all the ordinances of the ceremonial law, of which the passover was one of the earliest and chief. That type was laid aside, because now in the kingdom of God the substance was come.