De-icing is in most cases a decision taken by the pilots at the time as per the current conditions. Taking off with ice or frost on the tops of the wings or control surfaces must be avoided at all times.
You are allowed a thin coating of hoar frost to be present on the fuselage for take off. Thin meaning it melts away immediately on hand contact and you can clearly define the paint scheme etc through the frost.
You are also permitted to depart with ice, commonly up to 1/8th of an inch, on the underside of the wings due to cold soaked fuel.
In many cases the airline engineers will de-ice the aircraft prior to departure. The pilots turn up at the gate, find the aircraft already de-iced and can get on with the boarding straight away.
You may also see aircraft on the ground after their last flight of the day, being anti-iced. This is where they are sprayed with a type of de-icing fluid which stays on the aircraft surfaces and prevents ice or frost from forming.