onEnterFrame() and _currentFrame, help please

I have an onEnterFrame in frame 1 which contains a function to fade in my MC. My MC fades in.
I want that my MC fades out in frame 50.
Do I have to go to frame 50 and put a keyframe and another onEnterFrame in frame 50 ? Or is there a way to control this by AS? I mean saying that myMC._currentFrame is 50?