Early - mid September is the nicest month of all at Koh Samui. Not too hot, warm days, the odd shower here and there, which just cools things down. Plus it's low season so tourist numbers are down and prices lower. If the wet season comes in early then towards the end of Sept' rains will increase, but generally the heavy rains don't kick in until Oct/Nov.
I'd rather go to Samui in Sept' than any other month.