Flight Incheon International Airport departs from Seoul Incheon International Airport at 18:40 and arrives at Tokyo Haneda Airport at 21:00.
The flighttime is about 2h 20m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 637 Km.
The best day to arrive at Seoul is Sunday and it would be best to avoid Thursday, as prices are on average 73.15% higher than on Sunday. (Average prices, based on 2637947 datapoints.)
We have a lot more statistics for flights OZ 108 and between Seoul and Tokyo.