Yawmuddīn refers to the day of Judgment. It is the day when Allah will judge all of mankind based on their deeds and determine their eternal fate. It is also known as the Day of Resurrection, Day of Reckoning, and Day of Accountability. This day is described in detail in the Quran and Hadith, and Muslims believe that it will be a day of great trial and tribulation, as well as reward and punishment.