Official start of Halloweentime hasn't been announced so it's hard to know. But end of August it will definitely be down. Last year they did start Halloweentime the Friday of Labor Day weekend so if they do that again then it would open with that (which could include your "beginning of September" timeframe) but who knows. Last year was the first time in recent memory for me that they started Halloweentime that early.
Personally, I think the current CA Resident ticket offer expiring on 9/15 suggests Halloweentime might start 9/16, though that seems a little late.