CHR Comment: The article proposes that the Last Supper was not a Seder because of potential differences between the record from John’s Gospel and the record provided by the other Gospels. One should note, however, that there were different ways of describing time in the first century, which likely explains the differences in these accounts.
Today’s Seder meals and Haggadith are historically recent developments although they are based on the ancient Passover meal described in Exodus. Jesus and His disciples would have celebrated Passover according to practices in the first century, a time when devout Jews had a variety of practices, which may also contribute to confusion about the events that night. The best explanation is still that Jesus and the disciples observed Passover on the night when He was betrayed.
Source: The ‘Splainer: Was the Last Supper a Passover seder? | Religion News Service