'Monday' and 'Mondays' can both mean every Monday. 'Monday' is fine if the context makes it clear that every Monday is meant. In your example, because you're using present simple, it is clear that every Monday is meant. The question doesn't refer to a specific Monday. That would need a future tense. Actually, the issue is with the verb 'go'. If you said, "Do you have your dance class on Monday?" it would be understood to refer to the next Monday in the future. e.g. Do you have your yoga class tonight? - fine Do you go to your yoga class tonight? - strange

As Claire said, 'on Monday' can mean one specific future Monday or every Monday. However, I usually recommend that my students say 'on Mondays' if they mean every Monday, just to avoid misunderstandings. By the way, if you use present simple to talk about the future, this is not casual, it's very normal grammar. In English, we can use present simple to describe future things if they are scheduled. For example: When does your flight leave? The movie starts at 19:00. The new product comes out next week.