Albert Barnes on Matthew 26:20

When the even was come. The lamb was killed between the evenings, Ex 12:6, (Hebrew) that is, between three o'clock P.M. and nine in the evening. The Jews reckoned two evenings, one from three o'clock P.M. to sunset, the other from sunset to the close of the first watch in the night, or nine o'clock. The paschal supper was commonly eaten after the setting of the sun, and often in the night, Ex 12:8.

He sat down. At first, the supper was eaten standing, with the loins girded and their staff in their hand, denoting the haste with which they were about to flee from Egypt. Afterwards, however, they introduced the practice, it seems, of partaking of this as they did of their ordinary meals. The original word is, he reclined; i.e., he placed himself on the couch in a reclining posture, in the usual manner in which they partook of their meals. Mt 23:6. While sitting there at the supper, the disciples had a dispute which should be the greatest. Lk 22:24 and Lk 22:25-30. At this time also, before the institution of the Lord's Supper, Jesus washed the feet of his disciples, to teach them humility. Jn 13:1 and Jn 13:2-20.