Flight Haneda Airport departs from Tokyo Haneda Airport at 01:30 and arrives at Seoul Incheon International Airport at 04:10.
The flighttime is about 2h 40m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 558 Km.
The best day to arrive at Tokyo is Sunday and it would be best to avoid Saturday, as prices are on average 433.52% higher than on Sunday. (Average prices, based on 1328452 datapoints.)
We have a lot more statistics for flights OZ 177 and between Tokyo and Seoul.