They really are working weekends. Received my conditional approval today, letter dated 7/31!
I'll try to run through the more notable interview center times in the rest of this post. Note I won't be keeping this post current, so consider this a snapshot overview as of 7/31. Of note, Mondays seem to be quite popular everywhere:
Seattle is full until January 2017
Sweetgrass, MT is full for August, not available until September 6
Derby Line, VT not available until mid-November
Detroit not available until next week
Halifax not available until mid-September
YUL not until mid-August
YOW not until beginning of September
YYZ is available for a few times this week, but sparsely after that
All others not mentioned specifically are relatively widely available for August and onward. Some (e.g. Sault Ste. Marie) are practically wide open.