I couldn't find precise information on when the boost will be available, but I assume it will be on launch day.
For those that didn't play Classic, a problem that will occur is that players will head into Outland without any professions and be forced to spend time in the old world after you hit 70. This is not great if you come back for TBC to play the actual expansion.
If they let people use the boost prior to the launch day, that will leave players some time to catch up with professions. They'll already be much behind others due to the lack of gold and such, so there's already a large advantage for players that have lvl 60 toons ready.
On the other hand, people will have the opportunity to level up their blood elves and draenei before the actual launch day. It could be appropriate for the boost to be available on that time too.
When would be the perfect timing for the 58 boost for you ?