If he does it earlier, Hillary would have to step down and be replaced by . . . a very popular Bernie, or a very popular (barf) Joe Biden. The masses would rally to them as saviors from the evil Republicans.
So, maybe doing it "in stages" is the best way!
So holding the information until that deadline hits is smart; it prevents a replacement of Hillary! with anyone else. And it is close enough to the election that it WILL be in the news and front-and-center in the debates, but still before early mail-in voting (which is usually 30 days prior).
The best time to drop this kind of stuff is September - after the ballots are locked, and before mail-in ballots are cast. That's the time to unleash the dogs of war...
Before the first debate would be optimum. Early voting starts in October.
Doing it in stages may be okay. He needs the big hit to take place just before the debates. If they ever occur. No time for the MSM to prop her up.