“And he said, Go into the city to such a man, and say unto him, The Master saith, My time is at hand; I will keep the passover at thy house with my disciples.”
Matthew 26:18 — Greek Interlinear
Church Fathers on Matthew 26:18
The Evangelist having gone through the events preliminary to the Passion, namely, the announcement of the counsel of the Chief Priests, and the covenant for His betrayal, prosecutes the history in the order of events, saying, 'On the first day of unleavened bread.'
The first day of unleavened bread is the fourteenth day of the first month, when the lamb is killed, the moon is at full, and leaven is put away.
And observe that with the Jews, the Passover is celebrated on the first day, and the following seven are called the days of unleavened bread; but here the first day of unleavened bread means the day of the Passover.