If you are going to be absent due to religious observance, you should give your principal sufficient notice so that he or she can hire a substitute teacher for you. Your school secretary has the appropriate form for you to fill out. In many instances, your principal may provide you with an accommodation to help satisfy your religious observance (see Chancellor's Regulation C-606). If your principal denies you either an accommodation or the day, you should immediately speak to your chapter leader, who can file a grievance.