If your employer can demonstrate undue hardship, it does not have to accommodate your religious practices. One way employers can show undue hardship is if changing the seniority system to accommodate one employee's religious practices denies another employee the job or shift preference guaranteed by the seniority system.
If this is the case in your workplace, you may wish to speak with your coworkers to see if someone will trade shifts with you voluntarily, ask your employer if you can make up the work at other times, or transfer into another position that does not require that you work on the day of your religious observances.