I've only had a quick look at the pic, but Maurice Speed did the film review books, this is most likely a poster from a book shop to advertise an up coming book on the film Halloween. As these could be from any time period, depending on when the book was released, the date of 2000 if it's stated on the poster will be the correct date. I have a few book release posters myself, they are not part of a films campaign, just an ad for a book release.
If it was a 36x24 it would definitely be a commercial poster, but at that size could well be a store ad.
I'll have a bit of a dig if more info is needed?
Having a quick look around, it maybe a poster put out by Film Review, to make a few extra bob..
Either way it's a Commercial, and most likely worth a couple of Quid.