British Airways modifies 1 flight per week out of YYC

Beginning Oct 25, British Airways flights between Calgary and London will operate at different times on Tuesdays only. For the remainder of the week the flights will continue on their current schedule.

Flight times as follows (Tues only)
YYC-LHR Dep 2015 Arr 1220+1
LHR-YYC Dep 1455 Arr 1725

Source: Airline Route Updates