The law on religious accommodation is actually treated the same as the ADA and it's requirement for accommodation. An employer is not required to completely change the nature of a job in order to accommodate a request for accommodation.
If working on Saturday, for this position, is a bona fide occupational requirement, then the employee can be required to work on Saturdays or lose the job. The real issue then is whether or not it is an actual occupational requirement. Where some companies have issues with this is when they schedule work on Sundays, and it is shift work, meaning they don't need everyone there, but the employer refuses to work with those that want to take Sunday's off for religious services. That's problematic, because the employer has other employees, other shift workers, etc. and the appearance then is a refusal to adjust due to religious discrimination.
You don't appear to have those facts here. You are only open the 5 days. This isn't a shift situation where some employees are off and others work....and even if it was, she's a manager.
I will say that if there is an accommodation that you can reasonably come to, you should. If there are multiple managers, you can have them rotate Saturdays and then have to work additional hours on the 4 days. I wouldn't just deny it out of hand. I'd ask what reasonable accommodations she might suggest which give you management during Saturdays, has her work the hours she is supposed to, and doesn't involve her just never working Saturdays anymore.
The reason I suggest that is because if you allow anyone to not work Saturdays, ever in the future, that will be latched onto as evidence of discrimination. You should at least have the discussion with her about what a reasonable agreement could be. If she stands by her "I will never work Saturday" then you're ok to terminate her.