Year 1666 (MDCLXVI) was a common year starting on Friay (link will display the full calendar) of the Gregorian calendar (or a common year starting on Monday of the 10-day slower Julian calendar). Often called Annus Mirabilis, the year 1666 is mentioned in the poem Annus Mirabilis. This year also saw a great advance in mathematics and physics by Isaac Newton. The year is also known for having all the Roman numerals, used only once, in order from biggest to smallest value.