Under federal law, an employer may not discriminate on the basis of religion. Therefore, it is required to make "reasonable accommodations" for an employee's religious beliefs unless doing so would place an "undue hardship" on the employer. Accordingly, if a company/business can show that accommodating a worker's religious practices requires more that basic administrative costs, diminishes workplace efficiency, infringes on other employees' job rights, impairs safety, conflicts with another regulation, etc., then it need not make the accomodation. Conversely, however, if your employer can change your work schedule without causing it an undue burden, then it should give you the requested time off. If you feel that your employer's refusal is unreasonable, then you have the right to file a complaint with the EEOC.