https://www.workplacefairness.org/religious-discrimination

10. Can my employer prevent me from taking off on religious holidays or my day of worship?

You should start by letting your employer know that there is a conflict between your religious observances and your work schedule. When your employer's workplace policies interfere with your religious practices, you can ask for what is called a "reasonable accommodation": a change in a workplace rule or policy which would allow you to engage in a religious practice without conflicting with your work obligations.

Your employer is required to provide you with such an accommodation unless it would impose an "undue hardship" on the employer's business, defined as an accommodation that is too costly or difficult to provide. The Equal Employment Opportunity Commission (EEOC) who oversees these types of claims, has interpreted an undue hardship to mean anything more than regular administrative costs, anything that reduces workplace efficiency or impairs workplace safety, anything infringing on other employees' job rights or causes those said employees' to carry the accommodated employee's share of burdensome work, or if the proposed accommodation conflicts with another law or regulation. Thus, employers are obligated to try in good faith to resolve the religious conflict, or identify an actual monetary or administrative expense. It is important for you to work closely with your employer to find an appropriate accommodation.

If the accommodation would impose a burden on the employer that cannot be resolved, the employer is not required to allow the accommodation. Many accommodations, however, do not require any monetary or administrative burdens. Whether your employer can accommodate your religious practices will depend upon the nature of the work and the workplace. Usually, your employer can allow you to use lunch or other break times for religious prayer. If you require additional time for prayer, your employer can require you to make up the time.

Employers must give time off for the Sabbath or holy days except in an emergency, unless the employee works in key health and safety occupations or the employee's presence is critical to the company on any given day. This time off does not have to be paid, however. If employees don't come to work, employers may give them leave without pay, may require the amount of time to be made up, or may allow the employee to charge the time against any other leave with pay, except sick pay.