The 8th of DHUl-Hijjah is Yawmut-Tarwiyyah because

The 8th of Dhu al-Hijjah is known as Yawm al-Tarwiyah because it marks the day when the pilgrims in Hajj prepare themselves for the main day of Hajj, which is the 9th of Dhu al-Hijjah, also known as the Day of Arafah. Tarwiyah means "to quench thirst" or "to water", and it is named as such because in the past, pilgrims used to fill their water containers in Mina on this day in preparation for the day of Arafah. On the 8th of Dhu al-Hijjah, the pilgrims put on the Ihram clothing and proceed to Mina, a small town near Mecca, where they spend the night in tents. This is where the pilgrims rest and prepare for the rituals of Hajj that will take place the following day. Although no major Hajj rites are performed on the 8th of Dhu al-Hijjah, it is a significant day for Hajj pilgrims because it marks the beginning of the Hajj rituals, and it is a day of spiritual preparation and reflection for the pilgrims before the Day of Arafah. The pilgrims spend the night in Mina in preparation for the next day, where they will head to the plains of Arafah to perform the most important Hajj ritual, which is standing on the mountain of Arafat in supplication to Allah.