No holiday for the server

Sports Bar is closed on Thanksgiving so I had the day off yesterday. Lucky me, that translates into two 15 hour doubles over the next 48. Actually, 39 hours.
I have a problem saying no. A few weeks ago the Saturday night server asked if I would work for her this weekend. I knew that the Saturday bartending shifts were close to being mine, but I said yes anyway. Why did I do that? I don't want to work two doubles in a row,let alone a Saturday karaoke night. I said yes because I feel guilty if I say no. In the restaurant industry we get no paid holidays or vacation days and only having 3 servers makes the shifts hard to cover. I have this strange idea that if I don't have plans, I should be working. Great for making money, but not so good for my health sometimes, and certainly not for my social life.
Some people call this a good work ethic; being on time, covering shifts, not calling in sick. I call it a problem. The root of the problem I do not know. One thing I do know is I need to start saying no to picking up shifts. I already work 7 shifts a week, 6 days. Its reasonable not to want to work extras after all that.