The fuel your plan says you must carry will vary, depending on weather or other factors. Usually you're planning to have 45 minutes overhead your alternate airport (if one is required) as a minimum. When you get on the plane and close the door you have at least this minimum fuel on board (more is nice, but sometimes not possible) and are "released" to a certain place. What you do with the fuel to get you to the point where you are "released" to is entirely up to you (the Captain that is), arriving overhead and encountering the situation given here:
If you leave with legal minimum fuel, you have NO holding fuel at destination. So if ATC ask you to hold 15, 10, even 5mins you have to divert straight away.
Is (alone) not a good reason to divert to another airport. The fuel is not intended to be used for holding, it is fuel that must be carried and used at the Captains discretion.