I should have pointed out that not all churches observe All Souls Day in October. The Orthodox for instance observe it in June, I think.
But the mainline Protestants as well as the evangelicals do not - it would be an insult to them if someone claimed they did. It doesn't have a biblical mandate - in fact, some may even argue the opposite. So.. if it isn't observed by all of Christianity, the I don't think it can be called Christian. It seems to be more of the liturgical, ritualistic churches that pay attention to it. And that isn't all of Christianity.