I think you can just set your pc to the halloween date to trigger the event though.

Does Cucurbito the Pumpkin from Serious Sam: The Second Encounter count as Halloween content?

true :)

Haven't seriously played the game yet, but Hollow Knight got an update at one point (might have been a year ago or something?) that added some Halloween themed content.

Shovel Knight has one stage (Specter Knight's stage) that could be considered halloween'ish; in Specter of Torment, you even get to play as that stage's boss.

Dust: An Elysian Tail has an entire chapter (the longest in the game) where you go through a haunted forest, fight hoards of undead, and explore haunted mansions.

Night in the Woods and Costume Quest, as others already suggested, provide some great lighthearted seasonal fun.

I bought this on Steam recently: https://store.steampowered.com/app/425650/Halloween_Forever/

Haven't tried it yet, but it has very positive ratings. Steam has some more Halloween themed games, but I don't know if any of them are good.

There was pumpkin night mod for half life 2 but steam updates broke old mods. I think there was a workaround.

CS:GO is currently peddling some halloween content, if that is what you call it.

They skinned chickens in dodgy "ghost" costumes, and skinned footballs as pumpkins and put them effing everywhere. I'm actually playing worse because the chickens are distracting me and I can't help shooting them.
DIE! CHICKEN, DIE!
And that crappy "turn into a spirit with a phantom knife" when you die...*headslap*.
It would have been more interesting if they had a dedicated halloween-themed map complete with zombie player skins instead.

Not on Gog :

Cursed Fates: The Headless Horseman Collector's Edition

Saw this thread too late, unfortunately, but maybe bookmark this for next year. This is about good user made modules though, so naturally you'd need to own the according games in order to enjoy these (but now you have a whole year to look out for discounts on them). ;)

There's a cute Neverwinter Nights Module with a Halloween theme. It's <1 hour, IIRC, only consists of puzzles, free exploration and dialogues, and might be suitable for kids (if not for some allusions to Jack the Ripper or other scary stuff):
Skullington Falls by Sharon Tipton

There are also a few cool Halloween levels for Fight the Dragon (Steam only), that can be found in-game when you know which titles to look for, e.g.:
- Trick or Treat!, by Zeph Grey
- Halloween Havoc, by Astranon

Not directly Halloween themed but an entry in a past Halloween Contest and with a creepy autumn mood is Harvest of Sorrows by Dan Crawford, for hacked Forgotten Realms Unlimited Adventures (Forgotten Realms - The Archive: Collection 2).
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There was a Halloween event in Dying Light Bad Blood.

Couldn't agree more. That game has such an amazing atmosphere :)

Dead by Daylight, though the pc version is steam only.

Vertical Drop Heroes has a Halloween- themed DLC.