I've just learnt about the Boryeong Mud Festival, which sounds kind of cool. Unfortunately, it's held in late July (21 July to 30 July in 2017), which Wikivoyage describes as being fairly hot and humid, which is fine for the mud festival but not for other tourism.
Does South Korea have any mud festivals during autumn? They should be suitable for adults, as opposed to mainly aimed at children. If I have to define a "Mud festival", it'd be getting muddy in some way that's more chaotic than merely receiving a mud facial.
This forum post suggests that there's other festivals, but doesn't give any details:
There are other mud festivals in Korea, but they don't carry the same image as a drunken frat party. They also don't attract foreigners. Coincidence?
Theoretical answers that mud festivals around the world are usually in summer, and therefore unlikely to occur in autumn in Korea, are welcome.