A day with 0% is treated as a no-spend day. On that day, the AI sets all managed campaigns to the minimum daily budget and doesn't scale anything. The system prefers reducing to the minimum over pausing, because pausing and reactivating campaigns every day resets Meta's learning phase and hurts performance. Important: since the campaigns stay active at the minimum, Meta may still spend that small amount. You can't bring a day's spend to exactly zero without pausing the campaigns manually. If you want zero spend on a given day, pause the campaigns yourself on that day.