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Here is a useful mnemonic diagram for remembering the exact year-date of the fire that destroyed the old city of London in 1666:

The 6's in the year 1666 look like smoke
rising from neighbouring houses on fire.

The Great Fire itself started at a bakery in Pudding Lane (now the site of the Monument) and destroyed most of the City of London over 3 days. Its growing effect was described in Samuel Pepys's famous diary.

Incidentally, when written in roman numerals, the year 1666 uses every roman numeral just once, in descending order - MDCLXVI !