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 370 datapoints.)
The best day to arrive at Bangkok is Monday and it would be best to avoid Sunday, as prices are on average 45.76% higher than on Monday. (Average prices, based on 370 datapoints.)
The best time to fly is at around 6. (Average prices, based on 683 datapoints.)