Please be aware that Seoul has multiple airports and that these results are for all airports serving the city.
The best month to fly between Seoul and Jeju City is April and the most expensive month to fly is May. (Average prices, based on 578 datapoints.)
The best day to arrive at Seoul is Monday and it would be best to avoid Friday, as prices are on average 198.48% higher than on Monday. (Average prices, based on 1831 datapoints.)
The best time to fly is at around 13. (Average prices, based on 1386 datapoints.)