They can not. You will want to let your employer know in advance of any days that you observe for religious purposes. It helps if you give them this information in writing, so that you can save a copy in case religious discrimination takes place.
Your employer is required to make those accommodations, as long as they are reasonable. This includes time off for the Sabbath or other observed holy or religious days. You may want to make up the time or work out some other situation with your employer. It is important that the two of you communicate beforehand.
At the end of the day, your employer is not allowed to stop you from observing religious days, no matter the religion you are a part of.
Undue Hardship
If your employer can demonstrate undue hardship on the days you need off, they are not required to accommodate your religion. For example, if your employer does not give you a day off because no other employees are available to cover your shift, this would not count as religious discrimination. However, they will need to inform you of their reasoning.
After hearing their reasoning, you will want to consider whether or not it sounds truthful. You may want to consider talking with your coworkers to determine if no one actually has off. There have been cases of employers faking undue hardship in order to not give someone off for their religious holiday. This would be considered religious discrimination.