Mark 15:6

Now at that feast Rather, at festival time. There is no article in the Greek (or in Luke 23:17; Matthew 27:15), and the apparent limitation of the custom to the Feast of the Passover is not required by the original words, or by the parallel in John 18:39. It seems to have been a custom, the origin of which is unknown, to release to the people on the occasion of the Passover and other great Feasts any prisoner whom they might select. The custom may have been of Jewish origin, and had been continued by the Roman governors from motives of policy. Even the Romans were accustomed at the Lectisterniaand Bacchanaliato allow an amnesty for criminals.