Most campgrounds in Maine close on Columbus day which is October 14 this year. (actually the Tuesday morning after). Some close by mid September so it would be good to call those you are interested in. If the campground has a website it will be posted but many of the "campground guides" will have the wrong date(s). In late September or October you probably do not need a reservation. We made a trip to Maine last year and one campground we called for a one night stay on the way further North said just come and pick a site, we are not very busy.