Among Us Halloween works differently from other Halloween events in games. Innersloth decorates their maps with pumpkins and cobwebs, and there's usually a special set of 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.
6 October 2021 -
It's now officially Halloween in Among Us! The game has enabled some of the more spooky items like Halloween Hats. The Skeld and MIRA HQ maps will get a decorative update later this month.
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 Innersloth has bigger plans in mind for this year as well, considering how popular the game has become. With October approaching later this week, we're hopeful for an update soon.