Flight Haneda Airport departs from Tokyo Haneda Airport at 01:30 and arrives at Seoul Incheon International Airport at 04:30.
The flighttime is about 3h 00m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 496 Km.
The best day to arrive at Tokyo is Wednesday and it would be best to avoid Sunday, as prices are on average 190.89% higher than on Wednesday. (Average prices, based on 1543802 datapoints.)
We have a lot more statistics for flights OZ 177 and between Tokyo and Seoul.