|Last Supper - Chartres Cathedral - Stained Glass|
The Seder is the traditional Jewish Passover meal, usually observed by families rather than religious communities, during which participants rehearse the story of the Hebrew people's escape from Egypt as described in Exodus. If one accepts the chronology of the synoptic gospels (Matthew, Mark and Luke), then the Last Supper at which Christ washed his disciples' feet and then instituted the Holy Eucharist was a Passover meal. Thus there is an understandable logic to linking the two sacred meals and to attempt to "bring them back together."
There are several problems with this narrative however, and the fact that the chronology of John's gospel places the crucifixion on the day of preparation for Passover (that is, on the day the Passover lamb is slaughtered for the meal) is not the most pressing one.
The first problem is historical anachronism. While most scholars accept that a ritual Passover meal almost certainly existed in early first century Palestine, it is generally accepted that the Seder as a particular and widely observed structure for that meal is a much later development, likely at some point after the destruction of the Temple in 70CE. Indeed, there is a suggestion from some scholars that the evolution of the Seder meal was, at least in part, driven by an intention to create a ritual meal more distinct from the Christian Eucharist. So, even if the Last Supper was a Passover meal, it was almost certainly not a Seder that would be recognizable by most 21st century Jews.
A second problem is the propriety of some of the symbolism if moved from a Jewish to a Christian context. The most obvious example of that would be the closing expression of hope, "Next year in Jerusalem," since that makes no particular sense in a faith that is not tied - or at least not tied in the same way - to the earthly Jerusalem. Of course, one could reinterpret this as referring to "Jerusalem which is above," but that clearly isn't what our Jewish neighbours are talking about.
Which takes us to the larger problem about appropriation of voice and ritual. Should we play-act (however innocently intended) the rituals of another faith community? Moreover, given the long history of anti-Semitism in Christian history, should the (historic) persecutors appropriate the rituals of their victims? Indeed, by placing the Seder in the context of Maundy Thursday and by incorporating a celebration of the Eucharist, are we not engaged in a deliberate act of supercessionism whereby we expressly and overtly invalidate the very rituals we are appropriating? Holy Week already has a long, sorry and disgraceful history of being associated with pogrom and persecution. Do we need to add to it by aping a ritual that is not ours?
While the parallel is not precise, Islam (which reveres Jesus as a prophet) arose from Judeo-Christianity in a way not unlike how Christianity arose from Judaism. I've never heard of a mosque holding a "Muslim Eucharist," but I have trouble imagining that most Christians would find that appropriate, especially Christians in Muslim majority countries where Christians face legal restrictions and periodic harassment.
Admittedly Jewish opinion on this is not monolithic, and there are many Jews who interpret the "Christian Seder," very charitably, as an attempt to understand Judaism better.
Two fairly solid blogposts lay out both sides of this argument in ways I think are generally fair and reasonable, here and here. An excellent discussion on the problem of the Last Supper as Passover meal can be found here.
None of this is to say that Christians should not participate in a Seder in an appropriate context - as a guest of a Jewish family, for example. It could be a useful bit of interfaith learning to invite the local Rabbi to speak about Passover and the Seder. Learn about the Seder and about Passover by all means, and about the experiences of our Jewish neighbours. Christians who are part of interfaith marriages, or Jews who have become Christians while retaining their self-identification as Jews would hold or participate in Seders as they deem appropriate.
All that said, a "Christian Seder" is not and cannot be a Seder in any meaningful sense. It can only ever be an appropriation of someone else's rites. For ethnic gentiles to appropriate the Seder strikes me as no different than white comedians in blackface, professional sports teams named after derogatory terms for First Nations people or the archetypical rude uncle who tells ethnic jokes. The only defence is that, like the rude uncle, we don't realize we're being offensive. And that isn't much of a defence, really.