It definitely could affect your supply, one day won't make much of a difference. But every day and it will most likely make a difference. BF is supply and demand, pumping less means your telling your body you don't need as much.
Some people it doesn't affect them, though. If you're already struggling with your supply, I really wouldn't risk it.
As far as night weaning goes, your body should adjust so you don't make as much at night. I wouldn't keep nursing at night because you're worried about your supply. You need your rest. I night weaned to just one feeding around 9-10 months and it didn't really affect my supply.