Flight Changi Airport departs from Singapore Changi Airport at 22:30 and arrives at Seoul Gimpo International Airport at 06:00.
The flighttime is about 6h 30m. The distance travelled ("as the bird flies"): 2961 Km. So the average speed is roughly 456 Km.
There is an 1 hour time difference between Singapore and Seoul
The best day to arrive at Singapore is Thursday and it would be best to avoid Sunday, as prices are on average 48.7% higher than on Thursday. (Average prices, based on 406932 datapoints.)
We have a lot more statistics for flights KE 644 and between Singapore and Seoul.