Flight Haneda Airport departs from Tokyo Haneda Airport at 19:50 and arrives at Seoul Incheon International Airport at 22:25.
The flighttime is about 2h 35m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 576 Km.
The best day to arrive at Tokyo is Thursday and it would be best to avoid Friday, as prices are on average 89.87% higher than on Thursday. (Average prices, based on 1487429 datapoints.)
We have a lot more statistics for flights KE 2104 and between Tokyo and Seoul.