My name is***** and I will be helping you with your question today. This is for informational purposes only and does not establish an attorney client relationship.
Illinois is an at-will state so an employer can file an employer for just about any reason at all. If you have an employment contract and your duties or defined as prn and part of your duties include coming in when needed on your day off then you might need to come in. Illinois law assumes that every employee is an at-will employee, but there are exceptions. If you have an employment contract, you may be guaranteed employment for a certain amount of time or perhaps there are only specific reasons why you can be fired; it depends on the contract. Other situations where an employee may not be at will is in union contracts or where some other form of agreement has been made between employer and employee.
In at-will employment, your employer can fire you for any number of reasons -- basically anything that isn't specifically illegal. For example, you can be fired for coming into work late, but you cannot be fired because you are pregnant. Note that just because you are pregnant doesn't mean you can't be fired -- it just can't be the reason for your termination. You cannot be fired based on the following: race, sex, religion, disability, citizenship, pregnancy, etc. It is illegal for an employer to base your termination on these factors. Also, an employee cannot be fired for filing a workers' compensation claim. Firing an employee for these reasons can lead to a wrongful termination claim.
So basically if you believe this situation could cause you to be fired, then this is not something that would be protected and if you are in fact fired, you would have little resource without an employment contract or union protection.
Please let me know if you have any further questions and please positively rate my answer if satisfied.