It is almost always done on both days.
You sign up for it once onboard at the Port Adventures desk.
I actually quite liked doing the 5K on the day we had the cabana, because I didn't feel like I needed to go back to the ship to wash up (which is what I normally do). I had the outdoor shower to rinse off under (can't recall if I wore my running clothes for that or if I struggled into my swimsuit for it), which is quite cool water and felt good. I personally found no *value* in the cabana (it was too expensive, and it's more expensive now, for anything you get with it, IMO) but that was ONE nice thing.
