The Last Supper was not a Seder as it was eaten on "our". Tuesday night which was erev Passover or the night of Passover. Remember Hebrew nights started a New day. The next day, Wednesday was Nissan 14/Passover when the lambs were slain in the Temple. These were the lambs to be roasted and eaten that night at the Passover Seder. Jesus was crucified ON PASSOVER just as the lambs were slain in the Temple therefore the meal he ate the night before, the Last Supper, could not have been the Passover Seder.
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.