Pennsylvania labor law

Is there an employment contract or union agreement that prohibits this action? Does it constitute some form of legally actionable discrimination? If not, then there is no limitation as to how many days/hours you can be scheduled or be required to be available for work (with the exception of truck drivers, pilots, some medical personal, etc.). The fact is that in an "at will" employment arrangement, a company can set the conditions of the workplace much as it sees fit. That all having been said, to the extent tht you are paif hourly, you need to be paid for all time worked, and to the extent that it puts you over 40 hours in your work week, you are also entitled to be paid at overtime rates.

Yes, an employer may require you to work or be available for work 24/7/365; there is no no limitation in the law on how much you can be required to work, or guarantying you any days off, and if you are not available when the employer wants, they can suspend you--or terminate you, or demote you/reduce your pay, etc. Their only obligations are:
1) If you are hourly, you must be paid for all hours you work.
2) If you are not exempt from overtime, you must be paid overtime when working more than 40 hours in a week.
The only exception to the above is if you have a written employment contract specifying your hours; if so, you can enforce the terms of that contract and they cannot discipline you for sticking to your contractual rights--if they do, you  could sue them for breach of contract.