Jamieson, Fausset and Brown on John 18:28

28. Then led they Jesus from Caiaphas to the hall of judgment--but not till "in the morning the chief priests held a consultation with the elders and scribes and the whole council against Him to put Him to death, and bound Him" (Mt 27:1; and see on Mr 15:1). The word here rendered "hall of judgment" is from the Latin, and denotes "the palace of the governor of a Roman province."

they themselves went not into the judgment hall lest they should be defiled--by contact with ceremonially unclean Gentiles.

but that they might eat the passover--If this refer to the principal part of the festival, the eating of the lamb, the question is, how our Lord and His disciples came to eat it the night before; and, as it was an evening meal, how ceremonial defilement contracted in the morning would unfit them for partaking of it, as after six o'clock it was reckoned a new day. These are questions which have occasioned immense research and learned treatises. But as the usages of the Jews appear to have somewhat varied at different times, and our present knowledge of them is not sufficient to clear up all difficulties, they are among the not very important questions which probably will never be entirely solved.