I have been shopping around for a one-way longhaul flight on AC, having never flown with them before. I found their fares to be a little exorbitant, however (at least as of now) and am quite frankly a little reluctant to pay that price for my travels come late August. Yet, when I searched for dates on the same flight for travel before July 15, the price miraculously drops US$350, even though some of the flights are even fuller than the august ones. Does anyone know why this is? Does AC release more seats closer to the date? Or is July 15 some special cross-over, peak season point? I really hope it's the former case, as I would love to give my business to them this time.

I'm not sure whether July 15th is a cross-over into peak season, but fares can change enormously depending on whether they are having a seat sale and on which fare buckets come open as people cancel or book. It's not unusual, for example, to see T+ fares that are higher than Lat. I'd keep checking to see if there are changes in the next little while.
Where are going, because that will make a difference?

Thanks... I will wait a while longer. But I'm kinda afraid too that the price would jump as I wait longer. Should I wait (especially in the light of the current virus scare), or is August always busy (I'm flying to YVR)?

from where?

If the one way "long haul" is intercontinental, then you are running into the fairly standard problem of not having access to the cheaper fare buckets. Could you provide us the flight itself (YYZ-LHR, or whatever the routing happens to be). The pre/post July 15 could be due in part to ongoing fare sales in the early summer but the routing really matters.

One other suggestion might be buying a round trip and just throwing away the return. Sometimes can be worth a considerable savings

I have tried looking for roundtrips. It's slightly more expensive than one-way so it doesn't quite work. The routing I'm looking at is HKG-YVR - so it's definitely intercontinental. So I guess there's no hope now for a cheaper fare?

I did a quick check on ITAsoftware for the month of July assuming you want a non-stop flight from HKG-YVR return, so my check would cover Air Canada and Cathay Pacific. When I use that direction there is a cheap flight leaving July 6th (used a 6-8 day trip period) and then after July 11th all flights are a consistently higher price.

Then I tried the opposite routing, YVR-HKG return non-stop, and you would get about the same fare ($1382 CDN all-in return) most every day in July.
It looks like the problem you are having is that your start point is Hong Kong, and both airlines are using a different pricing model for this departure city!

If you want more help, be more specific with your travel information!! But this was a surprising result for me so I understand your confusion - PM me if you want more detail as I can then send you screen shots of the Search results.
By the way, your best fare option is to fly Northwest Airlines routing through Seattle but again I was guessing that you wanted to fly non-stop.

Superstarry

Fares seem to stay stagnant pretty much... Am I too impatient? Or are passenger loads back up? - it's just that the current fare is too expensive for an 11 hour flight. I can get cheaper between Singapore and Vancouver!