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 Hanoi is September and the most expensive month to fly is August. (Average prices, based on 360 datapoints.)
The best day to arrive at Bangkok is Thursday and it would be best to avoid Wednesday, as prices are on average 81.6% higher than on Thursday. (Average prices, based on 355 datapoints.)
The best time to fly is at around 18. (Average prices, based on 589 datapoints.)