Not all Steam sites have announced early download. At least I did not see it, but I am not getting it from Steam. So, I may not be able to see the download schedule.
However, those that do have it (I have heard UK and Australia from the boards) the game will not activate or be playable until Midnight on the release day, well, morning of the release day.
In your case, since there is a pretty good chance the game is going to be larger than 5GB. Talk to your Network Gods and ask them if the counters reset at a certain time each day, like Midnight. if they do, then plan the download so it starts before the reset so it will finish after. Otherwise, the Steam Download manager should restart the download for you when it can. The Network folks could probably give you a strategy to get the download with the limitations that are in place.