Matthew 26:18

And he said, Go into the city to such a man, and say unto him, The Teacher saith, My time is at hand; I keep the passover at thy house with my disciples.

Dummelow noted that:

The Last Supper is here called the Passover, because in many respects it resembled it. It is not, however, certain that there was a lamb. Jesus himself was the Lamb; and, as he intended to supersede the type by the reality, it was not absolutely necessary for the type to be present.[8]

Christ did not say, "I will eat the passover," but that "I keep the passover." Moreover, he did not say, "The Passover is at hand," but that "My time is at hand." Again from Dummelow:

The disciples would doubtless be surprised at the proposal of Jesus to keep the passover a day before the legal time. The disciples were therefore instructed to give the reason, "My time is at hand." The meaning was, "My death will happen before the legal time arrives."[9]

[8] J. R. Dummelow, One Volume Commentary (New York: Macmillan Company, 1937), p. 709.

[9] Ibid.