A.T. Robertson Commentary Mark 15:6

"Now at the feast he used to release unto them one prisoner, whom they asked of him." — Mark 15:6 (ASV)

Used to release (απελυεν). Imperfect tense of customary action where Mt 27:15 has the verb ειωθε (was accustomed to).

They asked of him (παρηιτουντο). Imperfect middle, expressing their habit also.