Please be aware that Bangkok has multiple airports and that these results are for all airports serving the city.
The best month to fly between Bangkok and Koh Samui is October and the most expensive month to fly is July. (Average prices, based on 311 datapoints.)
The best day to arrive at Bangkok is Monday and it would be best to avoid Sunday, as prices are on average 55.86% higher than on Monday. (Average prices, based on 325 datapoints.)
The best time to fly is at around 12. (Average prices, based on 518 datapoints.)