While departure times might change every 6 months, the changes are likely to be minimal while the frequency is basically maintained. There is always the option to travel from Bangkok just to Surat Thani and to take local means of transportation from there to Ko Samui. See entries on Surat Thani for details. Departures are from mid-morning until late afternoon. Provincial Flights Ko Samui - Phuket Bangkok Airways has two flights daily from Ko Samui to Phuket.
Departures are in the morning and the early afternoon.
Flight time is 40min. Phuket - Ko Samui Bangkok Airways has two flights daily from Phuket to Ko Samui. Departures are in the late morning and the mid-afternoon. Flight time is 40min. Local Boats Ko Samui can be reached by ferry, speedboat and car ferry from Ban Don and Donsak.