Albert Barnes on John 19:31

The preparation. See Jn 19:4. The law required that the bodies of those who were hung should not remain suspended during the night. See De 21:22,23. That law was made when the punishment by crucifixion was unknown, and when those who were suspended would almost immediately expire. In the punishment by crucifixion, life was lengthened out for four, five, or eight days. The Jews therefore requested that their death might be hastened, and that the land might not be polluted by their bodies remaining suspended on the Sabbath-day.

Was an high day. It was,

1st. The Sabbath.

2nd. It was the day on which the paschal feast properly commenced. It was called a high day because that year the feast of the Passover commenced on the Sabbath. Greek, "Great day."

Their legs might be broken. To hasten their death. The effect of this, while they were suspended on the cross, would be to increase their pain by the act of breaking them, and to deprive their body of the support which it received from the feet, and to throw the whole weight on the hands. By this increased torment their lives were soon ended. Lactantius says that this was commonly done by the Romans to persons who were crucified. The common period to which persons crucified would live was several days. To compensate for those lingering agonies, so that the full amount of suffering might be endure, they increased their sufferings by breaking their limbs, and thus hastening their death.