Please be aware that Seoul has multiple airports and that these results are only for Gimpo International airport.
The best day to arrive at Seoul is Thursday and it would be best to avoid Friday, as prices are on average 150.38% higher than on Thursday. (Average prices, based on 381 datapoints.)
At Seoul the most flights depart around 18:00. (Hourly flights, based on 209 datapoints.)
From Seoul it is on average the cheapest to fly to Osaka and to Shanghai. (Average prices, based on 124 datapoints.)