My unconfirmed research has led me to believe that Passover can be celebrated the day before or the day of. Therefore, the 'last supper' could have been a Passover Seder. It would make sense because Yeshua followed the traditions of the Passover meal.
uhm...where was the lamb? Are you thinking about the law that said if you were unable to attend the Passover you could participate in one 30 days later?