What a load of ruff c***s , get a 500W floor standing halogen lamp, position it at the angle of " truth" on walls( for ceiings wack a 4" nail in and hang the lamp on to hyper exagerate the plastering work) .Trowel up with this, you may not get things spot on, but at least when the sun comes around, you know what to expect. .....
Another tip..consider the aspect of each room , determine how each wall and ceiling will be affected by sun light before you start plastering. If the sun comes in through a window and high lights the ceiling.Try to do you last two or three passes in the direction of the light,that is to say cross trowel into the window not at 90 degrees to it. Your work may not be perfect but blemishes will be less defined by the sun light.