The submission form automatically opened at midnight East-Coast US time, just as it was supposed to. We immediately got six subs, which led me to think everything was working fine, so I stopped watching it closely. And then the form closed itself again, and I didn't notice for 45 minutes. Argh!
So a bunch of people presumably showed up right after East Coast midnight, as they'd been told to do earlier—and discovered that the form said we were closed and to check back in 23 hours.
Turns out it was a file permission problem. I had tested this whole part of the system thoroughly, over and over again, but apparently not quite thoroughly enough. It's enough to make me believe in unit tests.
I've now fixed the problem, and added safeguards to (I hope) keep it from happening again, and tested it all a couple more times, and I think everything now works as it should.
Nonetheless, I'll keep a closer eye on things tomorrow night at East-Coast midnight.
And if any of you showed up a few minutes after midnight tonight and thought we'd already reached our limit for the day, I apologize. Bear with us as we iron the bugs out of the system.
(On the plus side for authors, these bugs have actually led to more authors being able to submit than would otherwise have happened, because they've meant I had to reset the counter after receiving several submissions. Still, that doesn't make the situation any less frustrating for the authors who were turned away.)