Mark 14:15

And he will himself show you a large upper room furnished and ready: and there make ready for us. And the disciples went forth and came into the city, and found as he had said unto them: and they made ready the passover.

They made ready the passover ... These were preparations necessary to the observance of the feast, but only certain of the total preparations were made, the proof of this being in the fact that during the ensuing meal, when Judas left, following Christ's commandment, "What thou doest do quickly," "No man at the table knew for what intent he spake this unto him, for some thought, because Judas had the bag, that Jesus said unto him, Buy what things we have need of for the feast (the passover)" (John 13:27-29). Therefore, the meal that followed that evening was not the passover meal, for the excellent reason that there were still some things needed, and as yet not even purchased, that would have been required for the passover.