You said they asked for ONE friday. Yet youre working every friday now. The easiest way out of this is to say you cant do every friday anymore but are happy to return to your original agreement of 1 friday a month.

I guess im just confused why you continue to agree to each friday night because technically speaking you arent obligated to say yes as long as you give them one weekend per your contract. They should be asking every time instead of assuming, once they reach their 1 friday limit.

They arent taking advantage if you are agreeing to the time. Thats the point. If they are just assuming youre free and not asking thats one thing. Its entirely different if you arent saying no if you have the option to do so.