Our office is open two weekends per month. Are you available to work weekends if needed?

Most likely, if working weekends were required for the position, it would have been posted in the job description. If working occasional weekends is required, you should be aware of that by the time you go to your interview, and it should not be an issue for you. If it wasn't posted in the job requirements, the interviewer is letting you know that it might be a requirement. To be considered for the position, you should answer that you are available.