The job posting should have specified the work schedule, so you are probably aware that you may need to work weekends, holidays, and on-call when required. The interviewer wants to see your flexibility and availability if hired. If you are flexible with your schedule, let them know. If you have a family obligation that will keep you from working certain days, let them know of the conflict and explain that you will try to make accommodations. However, the more flexible you are, the more favorable you will look as a candidate.