J3piper and Ted D, have nailed it.
This is one that always seems to draw a wide variety of opinions. But the rules as J3 described and found in ENR 1.7.3 are quite clear.
Important to read note 1, the 'concept' section in ENR 1.7.3 where it describes further how the rules should be applied.
Also note that this concept section says pilots should maintain maximum practical obstacle clearance.
So, you can descend below MDA by day ifvisual with required circling visibility, but you should attempt to maintain maximum terrain clearance while doing so.