People don't have the day off from work here in the US for it, so I wouldn't consider it a huge holiday either. It's OK because people can still celebrate it at night. This year it falls on a Sunday, so most people probably have the day off anyway. As far as origins, I'd say check good old Wikipedia for a start. Here's one thing Wikipedia says: "It has roots in the Celtic festival of Samhain and the Christian holiday All Saints' Day, but is today largely a secular celebration" (http://en.wikipedia.org/wiki/Halloween).
I'll probably end up dressing up like a human like I do every year. But seriously, probably the usual Halloween dose of horror movies.