Sixty-six years ago, on Feb. 6, 1952, the U.K.'s King George VI was found dead in bed from a blood clot. On that day, his 25-year-old daughter Elizabeth became the new British monarch.
Queen Elizabeth II wasn't formally crowned until June of that year, but the U.K. still marks Feb. 6 as the date of her accession to the throne.
Past accession anniversaries have kicked off yearlong Jubilees to celebrate Elizabeth's long reign — the Crown previously held Silver, Golden and Diamond Jubilee events to mark Elizabeth's 25th, 50th and 60th years as queen.
The occasion is also a solemn one, however, since it commemorates King George's death.
Rather than publicly celebrating her Blue Sapphire Jubilee marking 65 years on the throne last year, the queen spent the day in quiet reflection. She is expected to do the same this year.
Elizabeth started as a long shot to take the throne. Her father only became king once his uncle, Edward VIII, abdicated the throne in 1936. As King George's oldest child, Elizabeth then became heir apparent.
Queen Elizabeth's oldest son, Prince Charles, is currently next in line for the crown, followed by his oldest son, Prince William.
The 91-year-old queen is Britain's longest-reigning monarch.