If you don't have a sign contractors license, it's illegal for you to touch any sign that is electric.
If you do have one, all existing signs you are allowed to do everything from changing bulbs, to ballasts/power supplies, new wiring, replace timer/photocell mounting, replace signs, etc.
If no sign was ever existing (i.e. brand new building) you are allowed to mount the sign and hook up the electrical only if electrical is there to be hooked into. You cannot run a new circuit off the breaker, you cannot add a timer, etc. Once a certified master electrican does that, from then on, you have full control.