28 Days Later
Silense Of The Lambs
Alien
Aliens
Dawn Of The Dead (original)
I'd like to count Shaun of the Dead but it's really not a horror film.
EDIT: The Thing too
Don't know about full, naked days but parks have tried to get a record for most naked people on a coaster at once. Wikipedia claims that Nemesis holds the record with 32 people. A naked day wouldn't surprise me with Alton, they have had gay days and were going to have a muslim day, which was cancelled.