The World Federation One Stop Fiqh

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.