Normally, if it's a first gen product (like the first iPhone or the first iPad), Apple might announce it months beforehand, with actual availability being later. If the "launch" is an update to an existing product line (like the imminent MacBook Pro refresh), either the product just shows up in the store, or Apple has an event, then during the event the presenter says "available now." Apple can't afford to release the news too early, or sales of the current model will halt.
So to answer your question, when the MacBook Pro gets updated, you should be able to order it the moment you hear the news.