As you are visiting during the low season (or green season as many call it since it is also rainy season), most flights will not sell out in advance.
But that said, there is only one flight each day direct from Chiang Mai to Koh Samui, so if you plan to use that flight, then booking ahead might be a good idea. If you plan to stop in Bangkok enroute from CM to KS, then there are numerous daily flights to Koh Samui, as well as several flights to Surat Thani from where you can catch a ferry to KS.
All of the domestic airlines do have online booking, so as soon as you decide your date, you can book the flight before going to the airport (better than sitting around for hours at the airport if the early flights are full).
Credit:stackoverflow.com‘
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