Almost any major transcon between YYZ and YVR, YYC or YEG [and return] during the peak vacation travel periods [Christmas, New Years, various holiday weekends], and the rest of the year on Friday and Sunday afternoons, Monday mornings when business travelers are trying to get home.

Good for either bumps or op upgrades from the various posts made over the years. Though ex-YYZ and YVR, it is not always easy to take advantage of such circumstances since gate agents are too focused on getting the flight out to follow procedure to the letter: i.e. upgrade elites first. And tempers and stress don't always make them welcome even a voluntary bumpee.