Good formulation. “Given it’s Monday” can have two different meanings:
you learn that you will only be awoken on Monday, then it’s 50%
you awake assign 1⁄3 probability to each instance and then make the update
So it turns out to 50 % for both but it wasn’t initially obvious to me that these two ways would have the same result.
(Perhaps you’re thinking of this https://www.lesswrong.com/posts/EKu66pFKDHFYPaZ6q/the-hero-with-a-thousand-chances)