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 775 datapoints.)
The best day to arrive at Bangkok is Wednesday and it would be best to avoid Friday, as prices are on average 107.62% higher than on Wednesday. (Average prices, based on 789 datapoints.)
The best time to fly is at around 17. (Average prices, based on 1352 datapoints.)