Flight Haneda Airport departs from Tokyo Haneda Airport at 02:00 and arrives at Seoul Incheon International Airport at 04:35.
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 Monday and it would be best to avoid Thursday, as prices are on average 286.41% higher than on Monday. (Average prices, based on 1298469 datapoints.)
We have a lot more statistics for flights JL 5257 and between Tokyo and Seoul.