It is likely not legal, since employers are expected to make reasonable accomodations for religion. There are circumstances under which it could be legal, however, if the employer's own circumstances have changed so that they definitely need to you work on your sabbath; therefore, it would no longer be a "reasonable" accomodation to not work that day. For example: say an orthodox Jew is employed by a retail store. He does not work Friday afternoon and Saturday. However, due to a change in the economy, market, or customer base, the store, which formerly was open on Sundays (when our employee worked a shift to make up for  not working on his sabbath), is now closed on Sunday but has longer Saturday hours. Under those circumstances, it may be unreasonable to have an employee not work on Saturday, and the employee also may not be able to work enough hours to justify his full-time salary since Sunday is no longer available.So the short answer is, there is a good chance that what you describe is illegal, due to the obligation to make reasonable accomodations, but you have to look at the circumstances--a change in circumstances can make what was formerly reasonable unreasonable.