What is the ruling regarding very slight bleeding that occurs for one day, and outside of the menstrual period?

The fundamental principle is that any blood discharged from a woman's uterus, not associated with childbirth, is menstrual blood, unless proven otherwise. The minimum period of purity (taharah (definition)) between two menstrual cycles is thirteen days according to the Hanbalis, and fifteen days according to the majority of scholars. If blood appears after the minimum period of purity has passed, it is considered menstruation, even if it is a single gush according to the Malikis. The Shafi'is and Hanbalis, however, hold that its minimum duration is one day and one night, while the Hanafis believe its minimum is three days and their nights.