John Chrysostom

"Now when the even was come, He sat down with the twelve disciples." Oh the shamelessness of Judas! For he too was present there, and came to partake both of the mysteries, and of the meal, and is convicted at the very table, when although he had been a wild beast, he would have become tame. For this cause the evangelist also signifies, that while they are eating, Christ speaks of His betrayal, that both by the time and by the table he might show the wickedness of the traitor.

Homily on the Gospel of Matthew 81 ↗

Theophylact of Ohrid

From this some believe that the Lord did not eat the Pascha that year. For they say that only while standing was the Paschal lamb to be eaten. Yet Christ sat down; hence it could not have been the Pascha which He ate. But we might say that first He ate the Pascha standing, and then He sat down and gave them His own Mystery and Sacrament. For having first kept the Pascha in type, He then kept it in truth.

Commentary on Matthew ↗

Jerome

And when evening came, he was reclining at table with the twelve disciples. Judas acts in such a way that suspicion of being a traitor is removed.

Commentary on Matthew ↗

Thomas Aquinas

Next the supper is treated: and when it was evening, he sat down with his twelve disciples. And it says evening, because, as is commanded in Exodus 12:18, the fourteenth day of the moon at evening you shall celebrate the Pasch. Or evening, because it tends toward sunset; Zechariah 14:7: and at the time of evening there shall be light. Or it signifies the true passing of Christ, namely the end: for at evening is the end of the day.

Commentary on Matthew ↗

Remigius of Rheims

With the twelve, it is said, for Judas was personally among them, though he had ceased to be so in merit. And it is beautifully said, When even was come, because it was in the evening that the Lamb was wont to be slain.

Catena Aurea by Aquinas ↗