Definitely check with the hotel front desk staff. Just a couple of weeks ago during Spring Break, I was with the family at the Crystal Gateway Marriott (Wash. DC). We were supposed to fly out on Saturday morning, but a storm came through Friday night and cancelled numerous flights. We were stuck until Monday afternoon in DC and the hotel was listed as sold out. Even though I am only Gold, the hotel not only extended my stay through Monday, they let me keep the same very low rate. ^

On the flight home, I talked with a Hilton Diamond member who was in the same boat with his family but at the Hilton across the street. They would only let him stay if he paid three times his current rate. (He decided to move a couple of miles away from DCA instead. )