ok, I just checked the schedule of airport express bus, they don't work on Saturday and holiday. there are 3 trips leave from Toronto airport to UW every day.
3.15pm, 6.15pm and 10.00pm, if you don't want to stay at airport, you can book door to door service, it will cost you $65 more. ($31 for general bus). you can check the service by 519-8862121, it's KW airport service