If you are part-time, you have to be available 3 times a week (includes one weekend day). I notice one week was closing shifts then the following was opening shifts, they seem to follow that pattern. You have to tell them your availability. Hours aren't always set. One day you work 4 hours, another you may work 8, depends on business needs. Unfortunately, if you need a day off, you have to give 2 weeks notice.
They will fire you if you cannot accommodate the business needs.
If you cannot accommodate their hours, you will be let go.