Unfortunately, there is no prohibition against employers requiring work on Sundays. If there were, Sunday would not be a major shopping day, and there would not be television on Sundays nor sporting events. The First Amendment grants US citizens protections for their religious beliefs. But that protection is from actions of the government
, not from a private business. A private business is not required to accomodate your religious practices.
The only protection one has from a private employer is that employer cannot to refuse to hire you because you are Christian, for example. That employer cannot decide not to hire any practicing Christians on the assumption that the Christians will refuse to work on Sundays (or Saturdays for those who have their sabbath then). That sort of discrimination based on one's religion is illegal.
However, that employer may require all employees, regardless of when they take their sabbath, to work on whatever schedule best suits the employer. There is no requirement that a private employer make any accomodation for any employee's religious observances and sabbaths.
An employee's only real remedy is to find employment that does not require work on their sabbath. You may want to discuss this with your employer before you resign, to see if you can negotiate a compromise of some sort, but unfortunately you don't have a legal remedy.
Good luck to you!
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Attorney at Law
Legal Disclaimer: The information I am providing you is for informational and research use only. You are paying me only for such information. The information is not legal advice and by rendering such information there is no formation of an attorney-client relationship. As the law is constantly changing, you are advised to consult with legal counsel in your area for specific information relevant to your situation.