I am Jewish, although my parents were not very religious. In the last year or so, I have begun attending services at the Temple again and studying the Torah. I recently decided to adopt a more religious lifestyle, including observing the Sabbath. I work a Tuesday through Saturday shift, so I spoke to my manager about making a change so I can take Saturday off. She told me that nobody wants to work Saturdays, and I can't get out of it. She also questioned why I suddenly decided to take Saturdays off, when I've been Jewish my whole life; she implied that I'm not being honest about my reasons for wanting the day off. Can she deny this request?
You are entitled to an accommodation for your religious beliefs, if your employer can provide one without undue hardship. So, from a legal perspective, the question is how difficult it would be for your employer to allow you to change your schedule.
Under Title VII, the landmark federal civil rights law, employers may not discriminate based on an employee's religion. Title VII prohibits employers from making job decisions, such as who to hire, promote, or fire, based on your religion. But it goes further: It also requires employers to make reasonable accommodations for employees whose religious beliefs, observances, or practices conflict with workplace rules, policies, or requirements. Employers must make such accommodations unless doing so would create undue hardship, defined as a more than minimal cost or burden on the employer.
In your situation, it seems clear that your religious observance of the Sabbath conflicts with the workplace requirement of working on Saturdays, imposed by your schedule. Although your employer has expressed some skepticism about the sincerity of your beliefs, The Equal Employment Opportunity Commission (EEOC), the federal agency that enforces Title VII, has pointed out that religious beliefs can change over time, so inconsistency between your past behavior (working on Saturdays) and your current request (wanting Saturdays off) should not necessarily count against you. Although an employer may deny a request that is not based on sincerely held religious beliefs, it typically needs more proof. For example, if you asked for Saturdays off last week so you could take a cooking class, then came back and claimed it was your Sabbath only after your first request was denied, your employer might have good reason to be suspicious.
Because your beliefs are sincerely held, the question is whether a schedule change would pose an undue burden. The EEOC has said that involuntary shift changes -- in other words, requiring one of your coworkers to pick up your Saturday shift, when that person doesn't want to do so -- is an undue burden. However, voluntary shift changes are a different story. Your employer might allow you to find a coworker who wants to swap days off, for example.
If it's really true that nobody wants to work Saturdays, and you can't find anyone willing to swap with you, you might be out of luck. Similarly, if the only way to cover your shift is for your employer to pay someone overtime or a shift premium, that would likely create an undue hardship. But if you are able to find a coworker who will trade days off with you, then your employer is probably required to allow it as a reasonable accommodation.