John Gill 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)

Now at that feast
The feast of the passover, which was at that instant; see (John 18:39) . The Syriac, Arabic, Persic, and Ethiopic versions read, "at every feast"; as if the following custom was used at every feast in the year, at the feasts of pentecost and tabernacles, as well as at the passover; whereas it was only at the latter:

he released unto them one prisoner, whomsoever they desired ;
of this custom (See Gill on Matthew 27:15).