Albert Barnes on Matthew 27:15

At that feast. The feast of the passover.

The governor was wont to release, etc. Was accustomed to release. From what this custom arose, or by whom it was introduced, is not known. It was probably adopted to secure popularity among the Jews, and to render the government of the Romans less odious. Any little indulgence granted to the Jews, during the heavy oppression of the Romans, would serve to conciliate their favour, and to keep the nation from sedition. It might happen often, that when persons were arraigned before the Romans, on charge of sedition, some peculiar favourite of the people, or some leader, might be among the number. It is evident that if they had the privilege of recovering such a person, it would serve much to allay their feelings, and make tolerable the yoke under which they groaned.

(s) "Now at that feast" Mk 15:6, Lk 23:17, Jn 18:39