Calmer weather/water of mid summer allows not only a nicer ride to that side of the boarder, but also allows a better drift. Halibut fishing seems to tail off "a bit" in the later half of August, but really there's fish there literally all the time. I'll be fishing the Canadian side out of Neah in mid-April (seas permitting of course), and we should have no problem getting our fish.
I agree with Jerry. Go in mid-late July or early August. (Chinooks too.)

J.D.