It starts... the period of the year with more sales ever lol.

The Halloween sale is live on itch.io (of course there'll be one even on Steam but can't say when it starts).

As usual older games are discounted more:

https://itch.io/s/21765/halloween-sale-2019



while more recent games less:

https://itch.io/s/21766/halloween-sale-2019