Have you gone to the Trick or Treat page? The instructions are given up at the top:
Welcome to the Trick or Treating Halloween event. You may choose one player to trick or treat with each hour. If you've received a trick or treat request, you will be notified via alerts. Once being notified, come back to this page and decide whether to give them a trick or a treat!
It says each hour, but that's old text from last year. It's actually every 15 minutes that you can ToT.