Koh Samui in May or June?

"What will the weather be like?"
That's the quintessential "silly question" for SE Asian travel I'm afraid. Very unpredictable weather in this corner of the world, but I'll take a stab at it. May/June is usually+ very hot and the start of the SW monsoon winds which bring turbulent and periodic thunderstormy weather to much of Thailand, but Samui being on the east side is more protected inside the lower Gulf of Thailand and +usually has good weather at that time.

However, just look at the current/recent unusually extreme rainy floody cool conditions in the south. This year has been most bizarre weather-wise so you might need to ask God for an accurate account of "What will the weather be like?"

Normally not much rain that area at that time. Humid? Yep, but less so than in the Andaman and other places in early wet season in the same period.
Unlike Bob and despite recent happenings I reckon Thai weather/climate is a lot more consistent than in mid latitudes.