Purity: Continuity and Calculation of the Three Days of Ḥayḍ
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 441:
It is not necessary for a woman to experience bleeding on the eve of the first and fourth day. However, the bleeding must not stop on the eve of the second and third day. Therefore, if from the start of the morning of the first day the bleeding continues until sunset of the third day and does not stop at all, it is ḥayḍ. The same applies if it starts during the first day and stops at the same time on the fourth day.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)