Yup. It's October, and Trick or Treat time is upon us. The floating island will appear in Croatoa later in the month, just in time for the Halloween Party on the 20th. There hasn't really been an announcement, outside of random in-game chat. You didn't miss anything.
Other than the candy canes, the Halloween salvage doesn't buy you anything... yet. It probably (maybe) will in the future, so stock up on Hammy costumes. There's a limit of 99, each.
Edit: to be clear, the 99 limit doesn't apply to the candy canes, just the Halloween costume salvages. Most of my chars are up around 700 canes, each.