Matthew 26:17

The first day of.. unleavened bread] As, according to St. Mark and St. Luke, this was the day on which the Passover lambs were slaughtered, it must mean the day before the Passover (Jewish reckoning), i.e. from sunset on Thursday to sunset on Friday. The last supper was held on Thursday evening, and the lambs were killed at 3 p.m. on Friday, but that would be on the same day, according to Jewish ideas.

In strict usage 'the first day of unleavened bread 'meant the first day of the Passover festival, which began with the paschal supper. But it is possible that the day before this, when the paschal lambs were sacrificed, and all leaven was expelled from the houses, was popularly spoken of as 'the first day of unleavened bread.'

The disciples came to Jesus] at or after sunset on Thursday, and within an hour or two the necessary preparations for the supper were complete. Where wilt thou that we prepare] 'For they might anywhere; since the houses at Jerusalem were not to be hired, but during the time of the feast, they were of common right' (J. Lightfoot). The rabbis say, 'It is a tradition that houses were not let for hire at Jerusalem, because they were not privately owned, nor were beds, but the householder received from his guests as a recompense, the skins of the animals sacrificed.' To eat the Passover] 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.

The paschal lamb was slain in the court of the Temple on the afternoon of the 14th Nisan, and was eaten the same evening after sunset, when the 15th Nisan had already begun: see Exodus 12, etc.