This article sets forth five rules with respect to what government may do to accommodate religious practice and five rules with respect to what government may not do. As it turns out the Supreme Court has said that most religious accommodations are left to the broad discretion of legislators and public officials. So long as the object of the accommodation is to protect or expand religious freedom, as distinct from expanding religion, the accommodation will be permitted.
Carl H. Esbeck, When Accommodations for Religion Violate the Establishment Clause: Regularizing the Supreme Court's Analysis, 110 W. Va. L. Rev. 359 (2007)