Monday marks the 60th anniversary of Queen Elizabeth's reign on the throne.
And that means it's the official start of her Diamond Jubilee celebrations.
Elizabeth became Queen in 1952 and to celebrate there are gun-salutes at Hyde Park and the Tower of London on Monday.
In the 60 years of the reign of Queen Elizabeth II, the country has had 12 prime ministers.
She became Queen so long ago, David Cameron hadn't even been born!
But it's also a sad day for the Queen, as it's the anniversary of the death of her father, King George VI.