Final answer:
On Christmas Day, 800 AD, Charlemagne was crowned as the Holy Roman Emperor by Pope Leo III.
Step-by-step explanation:
On Christmas Day, 800 AD, Charlemagne became the Holy Roman Emperor, a significant event in European history. This coronation by Pope Leo III marked the first time Western Europe was so united since the fall of the Western Roman Empire. Charlemagne's reign was notable for its expansion of territory, the promotion of learning and culture in what is known as the Carolingian Renaissance, and the establishment of a strong relationship with the church that would influence the politics of the era.