Which case involved the rights of Seventh-day Adventists to refuse to work on Saturday?

Prach v. Hollywood Supermarket, U.S. District Court for the Eastern District of Michigan, No. 09-13756 (2010)

Title VII of the Civil Rights Act of 1964 does make it illegal to discriminate based on Religion, it does not require a business to hire additional employees or make other employees work extra to accommodate a religious observance. This would be seen as an undue hardship on the business.