>> Awesome! Description's perfect, thanks. How soon should I make a post about the fest (explaining rules, etc) and the public card? <<

If the active period of the fest is meant to be October, then I'd say that advance announcements, rules, etc. could be posted in September. That would actually give people a month extra to start working on stuff, before posting opens.

If you'd like to do something sooner, there's plenty of room to do a spring or summer fest before October, with a different theme.

Oh, and I forgot one list, there is a prompt set for Horror Tropes too.