John Gill Commentary Luke 23:17

"[Now he must needs release unto them at the feast one prisoner.]" — Luke 23:17 (ASV)

For of necessity he must release one
And therefore proposed to release Jesus

unto them at the feast ;
of passover, which now was; not that there was any law that obliged to it, but it having been customary with the Roman governor to do so, the people expected it; custom had made it necessary.

And so the Syriac, Arabic, and Persic versions render it, "it was a custom to release" not at each feast, or every feast, as the last of these versions read, only at the passover, as is expressed, (John 18:39) .