John 13:29

Verse 29. Buy those things that we have need of against the feast] Calmet's observation here has weight so it. "The disciples who thought that our Lord had said this to Judas, knew well that on the day of the passover there was neither buying nor selling in Jerusalem. This, therefore, did not happen on the paschal evening; for the feast, according to the common opinion, must have begun the preceding evening, and Jesus have eaten the passover with his disciples the night before his death; but it appears to me, by the whole text of St. John, that the passover did not begin till the time in which our Lord expired upon the cross. It was then that they were sacrificing the paschal lambs in the temple. It is therefore probable that the apostles believed that Judas went to purchase a lamb, and the other necessary things for the evening, and for the day of the Passover." On this subject the reader is requested to consult the observations at the end of Matthew 26, where the subject is considered at large. See Clarke on Mt 26:75

Give something to the poor.] It is well known that our Lord and his disciples lived on public charity; and yet they gave alms out of what they had thus received. From this we learn that even those who live on charity themselves are expected to divide a little with those who are in deeper distress and want.