Its not really plot central, but this one shot, Harry rather crackily becomes headmaster and changes the Halloween celebration back into a more traditional Samhain kinda thing.
Oh, almost forgot your lack of FFN access, it's by Fringeperson, but I don't know if they're on AO3 or not, sorry.
I know I've seen others where older 'pagan' celebrations are brought back/renewed but can't think of any fic names
