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 November and the most expensive month to fly is August. (Average prices, based on 6592 datapoints.)
The best day to arrive at Bangkok is Wednesday and it would be best to avoid Sunday, as prices are on average 19.2% higher than on Wednesday. (Average prices, based on 6502 datapoints.)
The best time to fly is at around 17. (Average prices, based on 10306 datapoints.)