William the Conqueror was crowned King of England at Westminster Abbey on Christmas Day 1066. He was born the illegitimate son of Robert I, Duke of Normandy, and his mistress Herleva in 1027. 

His invading Norman army defeated King Harold II at the Battle of Hastings to take the English crown on 14 October 1066.

It is pretty definite that 1066 is THE most famous date in Britain.

The coronation of William, known as ‘the bastard’, merged French and English rituals. The rite of Dunstan was used for the first time at the coronation of King Edgar in 973, for William, it was read in English by Aeldred of York and in French by Geoffrey, Bishop of Coutances. William was then anointed with chrism, the sacred oil. 

Outside the Abbey, William’s knights were posted keeping guard against dissenters. When the knights heard cheers from inside the Abbey, they misinterpreted the reason and set about burning the buildings which surrounded the Abbey as a response to the perceived attack on their Lord.

The coronation was suddenly in chaos, the rejoicing congregation raced outside as smoke from the burning buildings entered the sacred Abbey, leaving the clergy to complete the coronation rituals for a trembling King William. 

Three years later, at Christmas 1069 William wore his crown and coronation robes in the ruins of York Minster as a symbol of his might against the northern rebellions that he was forced to quell. 

Sources:

https://www.thehistorypress.co.uk/articles/william-the-conqueror-crowned-at-christmas/

https://www.historyextra.com/period/norman/10-surprising-facts-about-william-the-conqueror-and-the-norman-conquest/

https://www.westminster-abbey.org/abbey-commemorations/royals/william-i-the-conqueror/