I think it absolutely has to be at a fixed predictable time before the event, no matter what, and it shouldn't be too far ahead of the shoot date.
I don't have a preference for any specific day or whether its morning or afternoon, but the current ad-hoc random unpredicable time really hacks me off.
We're missing Anston this year (one of my absolute favorite shooting grounds) solely because I didn't find out that booking had opened until it was already fully booked. And there's no way on earth I'm going to travel that far and pay for a night in a hotel without a guarantee of getting to shoot.
And another thing, I do plan to raise it with my region to propose that people pay the full shoot fee at the time of booking, with a refund policy such that the amount of refund diminishes the closer the cancellation is to the shoot. E.g. if you cancel two weeks in advance you get your money back, one week in advance you get half; cancel in the last week or dont turn up you forfeit the fee, no exceptions.
It is clear from the first two GPs that there were loads of people cancelling at the last minute or not turning up, even though the sessions are being fully booked within hours of going live (e.g. GP2 morning session was fully booked within 12 hours). It is clear that there are quite a number of people who are booking in at each GP just to reserve a place, not because they are certain that they want to go, but rather they are booking just in case they later decide to go. Many of which don't bother.
Every place booked by someone who doesn't intend to turn up is depriving someone else of a shoot, and considering that so many people travel so far and might also need to have booked local accommodation means that last minute cancellations cannot be filled by people who have missed out.