A device for determining appropriate injection or testing sites. At least two movable adjacent plates are each provided with holes arranged in a certain system that can be brought in coincidence. A date carrier plate has numbers corresponding to the days of a month arranged in a certain system. A time carrier plate has numbers corresponding to the various times, mostly the hours, or parts of a day. After moving the plates a coincidence of holes in each determines the appropriate date and corresponding time for the optimal needle insertion point for the given date and time. Site determination signs defining a more exact needle insertion point on the various body parts are also provided.