Probably when we were camping long before kids. The tent was so hot that she was naked in the tent, and all the windows were zipped open and the door to the tent was open while she lay on top of her sleeping bag totally nude. Sadly, no one could have seen her, even walking by, but she was so casual and calm and relaxed.
Not a nude moment, but close enough, was at our wedding rehearsal when she wore a dress that was a size too big without a bra and no underwear, flashing her nipples to most people there, seeing the fear on her mom and dad's face around the extremely modest and conservative family.
I actually brought that up to my wife the other day, and she is still mortified after the fact.