Do I call the hotel and tell them to add a day? Or do I book a new reservation and request a refund?
Depends, did you do it via OnPeak? If so, and you're adding a day like Sunday/Monday, then you should be able to retrieve your reservation and choose to add a day there. Now if you're trying to add like Friday or Saturday, there most likely won't be availability. As for calling the hotel, there won't have your reservation yet. Again, this info is based on the hotel lottery. If you got it another method, then disregard all of this