It is illegal to discriminate against someone for their practice of religion. If the job is open seven days a week then the employer obviously must have staffers for the full week and accomadating this fellow is just a matter of scheduling.
I ran into this years ago with a Hindu employer who insisted I had to work Christmas.
It cost him $7200 and legal fees.
I have a story for you. I am a claims investigator and I had a claim were a guy tried to change the date an estimate for auto repairs was written so it would read that it was written on 12-25 rather than 12-15.
I knew that the particular body shop in question was run by a Muslim, so I called the owner and asked him flat out, "Were you open on Christmas writing estimates?" He replied something to the effect of, "You've gotta be kidding? Christmas is a federal holiday. Everyone was off work that day."
The guy who fudged that estimate got in some trouble with the Dept. of Ins. for filing a fradulent claim.