Queen Elizabeth II becomes Britain’s longest-reigning monarch later when she passes the record set by her great-great-grandmother Queen Victoria.
The Queen will have reigned for 63 years and seven months – calculated at 23,226 days, 16 hours and approximately 30 minutes at about 17:30 BST.
Prime Minister David Cameron will lead tributes in the House of Commons and there will be a River Thames salute.
The Queen, who is 89, will spend the day on official duties in Scotland.
The exact moment the Queen reaches the milestone is not known because her father, George VI, passed away in the early hours of 6 February 1952.