Hi Kathleane! This is a brilliant strategy for boosting occupancy on those tricky shoulder nights, and June is the perfect time to test it out. To achieve exactly what you're looking for, a Rule-Set is definitely the right tool for the job because it gives you precise, surgical control over your calendar. You can specifically isolate the month of June, select only Sundays and Mondays, and apply your percentage discount to just those days without affecting your weekend rates.
Custom promotions, on the other hand, are fantastic for broad marketing pushes but are much more rigid, usually applying a blanket discount across consecutive dates or relying on 'Early Bird' rules rather than letting you cherry-pick specific days of the week. Just keep a quick eye on your minimum stay requirements when you set it up, if you have a strict weekend minimum, you'll want to ensure it doesn't accidentally block a guest from grabbing that discounted Sunday-to-Monday stay. Best of luck filling up your June calendar, it's an excellent way to maximize your revenue!