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.