If Steam is running and you are not currently playing a Steam game, most pre-orders will begin downloading at the appropriate time they become available. Most DLCs will download in the background even if you're playing a game; however, not sure how this works with expansions (especially since they moved this to DLC category under my Civ V base game).
But regardless, if you leave Steam running and go to bed (or work) it should download without requiring anything from you when the time hits.