If the interviewer is asking this question, then this position is probably going to requires some weekend work. Maybe it's not a lot, but they want to make sure that if something goes wrong and more work is required, you will be able to work on weekends. I recommend answer by saying it will not be a problem. If you want to find out how often you will have to work weekends, you can ask it after answering the question.
This answer is stating that it is no problem. It also supports the answer by saying I have worked on weekends in my previous group. And the last sentence is an example how you can ask them about weekend work.