It was quite a sunny day, mind you.
or in the middle:
It was quite a sunny day, mind you, so I packed my sunglasses.
Now mind you, it was quite a sunny day...
What is meant to be conveyed? That I should be mindful of the fact that was stated? I can understand that much, but somehow it doesn't quite fit in with my understanding of the word "mind" when used in other contexts: "Do you mind if I borrow your car?" or as a verb: "Mind the gap." (at subway stations between the train and platform).
Can someone break it down?