Final answer:
On Christmas Day, 800 AD, Pope Leo III crowned Charlemagne Holy Roman Emperor, symbolizing the union of church and state.
Step-by-step explanation:
Charlemagne was crowned Holy Roman Emperor by Pope Leo III on Christmas Day, 800 AD. This significant coronation occurred after Charlemagne had secured Leo's position in Rome and it symbolized the resurfacing of a Roman Empire in the West. The title was contested by the Byzantine Empire, but it solidified the alliance between the Frankish state and the papacy, framing the socio-political landscape of medieval Europe. This event set the stage for the complex relations between future emperors and popes.
The coronation highlighted the close relationship between the Carolingian dynasty and the Church and established the concept of the Holy Roman Empire, which endured for centuries as a political entity in Central Europe.