There is one option showing for AUS-YYC on May 20 through IAH....you just need to look a bit harder. Remember, when you see the white boxes on the Availability Calendar, it means you need to click on the date to check availability, not that there isn't any. And remember, long weekends are always a crapshoot, unless you book at the full 355 days out.

Alternatively, call the AP Contact Centre. You only have to pay if you book, so if they can't find anything you don't pay.

Of course, there's always someone on FT who will scold you to do your own research and then promptly give you the information you wanted, and today that person is me...