Chantipie is a really nice site high up overlooking the Loire. You should be OK in getting on without booking from 16th. We have been there about that time and there were pitches available.
The best area if you a lucky to get it, is a pitch actually overlooking the loire. I think there about 4 pitches in this area - If you want a pitch in this area I would try and book.
Site has quite a good resturant but there are several others in and around Saumur.
Been on this site several years for short stay and found it excellent