Well, one thing we can do to prevent people from complaining why they weren't able to sign up quickly is to announce the date and time of the sign-up time beforehand. So it's first come, first serve.
So...if we announce it beforehand....people will be able to expect when it will be and try their best to be there when the time arrives.
The time and date should be specific, but...because of timezones.....
Well, what we can do to overcome that timezone problem is instead of posting the date and time as it is, why not post it like:
THE SIGN-UP WILL START IN 3 DAYS AND 6 HOURS.
So that people will know when it'll start and expect it. Well...they just have to compute the time, of course.