Westminster Abbey's bells will peal, a flotilla will sail down the River Thames and a gun salute will ring out on Wednesday as Queen Elizabeth II becomes the longest-serving monarch in British history.
The queen herself is set to inaugurate a railway line in Scotland and host a private dinner at Balmoral Castle to mark the day she overtakes her great-great grandmother queen Victoria's record.
It is not known where exactly she will be at 1630 GMT, the best estimate from royal officials for the time at which the monarch, who has become synonymous with Britain itself, reaches the landmark.
But at that moment, the 89-year-old monarch will have served 23,226 days, 16 hours and roughly 30 minutes on the throne -- over 63 years.
Prime Minister David Cameron is expected to lead tributes in parliament to a figure best known internationally for her stoicism in the face of a slew of royal scandals, as well as for her colourful outfits.
"Her Majesty has been a rock of stability in a world of constant change," Cameron said in a statement.
"It is only right that today we should celebrate her extraordinary record, as well as the grace and dignity with which she serves our country."
The queen may deliver a speech for the occasion, according to media reports, although a royal source has said she wants to keep the occasion low-key because of the memories it evokes.
"While she acknowledges it as an historic moment, it's also for her not a moment she would personally celebrate, which is why she has been keen to convey business as usual and no fuss," the source explained.
The official photograph, taken by Mary McCartney, son of Beatles star Paul, shows the queen sat at her desk working through a red box of state papers.
Elizabeth became queen upon the death of her father George VI, Britain's king during World War II, whose youthful stutter inspired the Oscar-winning film "The King's Speech".
The calculation of her time on the throne is based on when he passed away, which is estimated at around 1:00 am on February 6, 1952 -- an hour after he was seen for the last time at his bedroom window at Sandringham House in eastern England.
The queen presided over a gradual decline in Britain's global influence as many of its former colonies became independent, as well as a sharp rise in living standards and the advent of the digital age.
"She has been on the throne so long, it's difficult to conceive of the country without her," said Judith Rowbotham, visiting research fellow at Plymouth University.
She has also steered the monarchy through some of its rockiest recent patches, including the collapse of three of her children's marriages and public anger at her reaction to the death of princess Diana in 1997, which some saw as cold.
The royal family has since tried to present itself as more in touch with the public.
That decision was crowned by the highly-popular marriage of the queen's grandson Prince William to commoner Kate Middleton in 2011, and the birth of the couple's two children, Prince George and Princess Charlotte.
As part of the celebrations, Royal Mail announced that first-class stamps would be purple for the next year while London's iconic BT Tower beamed the message "Long May She Reign" across the capital.
Most British newspapers splashed photographs of the queen on their front pages on Wednesday.
The Daily Telegraph's headline was "The longest to reign over us" while the Daily Express ran with: "Thank You Ma'am".
Historian Jane Ridley said it was "remarkable" that the queen had been able to maintain the monarchy's popularity, despite the global trend towards republicanism.
"That is an extraordinary achievement," she told the Times.
"She has done it very cleverly. She has never given an interview. She is constantly seen. There is a good quote from her: that 'I have to be seen to be believed'."
Most of her subjects remain fans. A YouGov survey earlier this year found she was the woman most admired by Britons, well ahead of actress Judi Dench in second place.
Telegraph pundit Allison Pearson said the queen could "with some justification" celebrate her achievements on Wednesday with her favourite tipple in hand -- a Zaza cocktail of gin mixed with Dubonnet.