The trial before Caiaphas ended in the early hours of the morning. The Sanhedrin (Jewish high council) did not have the power to execute someone. They needed Pilate, the Roman governor, to carry out an execution (18:31).
They did not want to become ritually unclean by contact with Gentiles in Pilate’s headquarters. So Pilate, probably fearing a riot, went outside to meet them.
The Passover meal itself had taken place the night before (see 13:1; Mark 14:14–16). The next day, another meal began the week-long Feast of Unleavened Bread (Leviticus 23:5–6).