Unless you have an outside obligation that keeps you from working weekends, you should avoid saying that you cannot work weekends. The interviewer wants to hire someone with a flexible schedule who does not mind working non-traditional hours. If you have something that prohibits you from working weekends, explain what that is and let the interviewer know that you can try to work around it. For example, if you are taking a class every Saturday, explain why you cannot work Saturdays, but with advance notice, you might be able to arrange something with your professor.
