Can my boss fire me for not wanting to work? Should I call back?

It's my day off from work. I work in the deli department of Highland Park Market. I am a very hard worker, and I go out of my way to please my manager. Whenever anyone needs me to cover their shift for them I always say yes; and when she asks me to change my hours from working until 2:00pm to working until 9:00pm I always say yes, even if it is an inconvenience to myself. However, today is my day off. And sure, I COULD probably go in to work today but I have other things I would LIKE to get done today. I need to go buy my father a present, cash my paycheck, pick up a prescription and do alot of chores today. All of this needs to be done early; and if I had to work I would probably get stuck having to wait until my next random day off to do these. (As the store I need to buy my father a present from is only open noon-five.) And to be honest; to go along with everything that needs to get done, I honestly just don't WANT to. Anyway, my question is, if I ignore my bosses calls can she fire me for not coming in? She said in the voice mail that they really need me. But I ALWAYS inconvenience myself for them. Is it wrong of me to not want to work today? Should I call her and make an excuse? Please let me know. Thank you. :)