If you've been fired over religious discrimination, you can feel guilty, furious, confused, and saddened. Being let go for missing work to observe a religious holiday is a form of religious discrimination, which is defined as being treated differently (often being harassed) due to one's religious beliefs, dress, practice, and yes, requests for days off for holidays. It's also considered religious discrimination if you're not granted a raise or promotion because of your beliefs.
Your employer must legally make reasonable accommodations and allow for you to practice your religion under Title VII of the 1964 Civil Rights Act. An employer's only defense is if your religion has created an "undue hardship" for the company - but these are easily avoided by asking a fellow employee to trade shifts with you, working overtime to make up for time lost, rescheduling meetings, or taking a temporary reassignment.
Though protection and specific laws against discrimination vary from state to state, most people in the private and public sectors are covered by Title VII as long as their company has 15 or more employees. Discrimination suits also extend to those who were not hired on the basis of their religion or those who are underpaid because they have missed days due to religious observance. Keep in mind that employers also can't schedule major meetings, trips, or interviews on holidays for current or prospective employees. As long as you ask for "reasonable accommodation," (for example, well in advance or with a solution for making it up or switching with a coworker in mind) your employer cannot legally deny you time off unless they can prove it is an "undue hardship."
If your request is not reasonable and/or your employer proves undue hardship, they are not required to accommodate you nor are they required to financially compensate you. However, there are many ways for an employer to make room for your needs: they can allow you to pray during lunch breaks, or make up time lost at a later date. Remember, it is in the employer's best interest to accommodate your needs - especially since if you were discriminated against, you certainly have a hefty case against your employer.
Though problems can arise with the seniority system, (like if your absence or shift would inconvenience someone with a higher title than you) most employers are smart enough not to jeopardize their reputation, or the loss of a quality worker, by committing religious discrimination. If you have been fired due to your religion, you should contact a lawyer immediately at 918-779-3658.