Praise be to Allah.
If the period ends for one day or one night during the days of the period, then the woman should do ghusl and offer the prayers that fall due whilst she is taahir (pure), because Ibn ‘Abbaas said: “When she sees the heavy blood, she should not pray, but if she sees the tuhr (white discharge indicating that the period has ended) for a while then she should do ghusl.”
Fataawa al-Lajnah al-Daa’imah (Majallat al-Buhooth al-‘Ilmiyyah, 12/102
The heavy blood is thick blood, or it was said that it means a lot of blood. Lisaan al-‘Arab, 4/46.