Team Fortress 2 website announces a new update is live in Team Fortress 2, saying: "In preparation for Scream Fortress 2014 we've enabled all previous Halloween events." A separate post adds more details:

Luckily, you can now play previous years' Halloween spectaculars to get your shock tolerance up in advance of the update. Simply boot up the game, hop into multiplayer and quickplay any of our past five Scream Fortress updates. If you've missed out on previous Team Fortress Halloweens, or are new to TF2 in general, this is a perfect time to catch up.