Among Us Halloween works a bit differently from other Halloween events in games.
Inner Sloth decorates a few of the maps with pumpkins and cobwebs, and there's usually a special set of Among Us Halloween hats and costumes available.
What makes these special is you don't have to do anything special to get them. All you need to do is play the game.
Among Us Halloween costumes and hats are available through the entire month of October. All you have to do is claim them from the costume select screen, and they'll remain in your inventory once the event ends.
However, there is a way to get these even after Halloween.
Among Us Halloween 2021 Event and Hats
If you miss the event or don't feel like waiting, you can adjust your system's internal clock to sometime during October and play the game to grab the costumes.
There's no penalty to doing so, other than whatever it could affect on your system (e.g., if you're playing Animal Crossing on the same platform).
As to what the Among Us Halloween 2021 costumes will be, we're not sure yet. It could be Inner Sloth has bigger plans in mind for this year as well, considering how popular the game has become.