It started as a religious holiday, but it got to be a sort of widespread celebration even for nonreligious people. I think it absorbed a lot of stuff from other festivals, like the tree thing.
I don't know what to call what has happened in my world. I suppose there could be different realities. I know there are things that have changed our future several times. And I can't say I'd be surprised if someone from one of those futures showed up here, but they haven't yet. I get the feeling that some of the encounters back home were from alternate futures, though.