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.
Business in the Commons will be postponed for half an hour so MPs can pay tribute to the Queen.
On the Thames, a flotilla of historic vessels, leisure cruisers and passenger boats will take part in a procession between Tower Bridge and the Houses of Parliament.
The bridge will lift as a sign of respect and HMS Belfast will sound a four-gun salute.
The BT Tower in central London is scrolling the message "Long may she reign".
Mr Cameron tweeted: "Over the last 63 years, Her Majesty has been a rock of stability in a world of constant change.
"It is only right that we should celebrate her extraordinary record, as well as the grace and dignity with which she serves our country."
Mary Francis, one of her aides in the late 1990s, said the very private monarch could also be "great fun".
"I've seen her on one of the last holidays on the Royal Yacht Britannia - the only time I ever saw the Queen wear trousers.
"She let her hair down. She told stories. She had everybody in fits of laughter so she can be very relaxed - at times."
Queens of the modern age
Queen Victoria became queen at the age of 18 and ruled for 63 years, seven months and two days.
Queen Elizabeth's reign has included 12 prime ministers, two more than served under Victoria.
One of those prime ministers, Conservative Sir John Major, rejected any suggestion the Queen had been too passive as head of state: "The monarchy wouldn't be as popular if they were part of politics - they're above and beyond it.
"But when the Queen meets her prime minister she has the opportunity to question, to ask, to counsel. Nobody knows and no prime minister is going to tell you exactly what happens at those meetings. So those who say she's been too passive, how can they possibly know?"
The Queen is sovereign of 15 Commonwealth realms in addition to the UK, and Brian Mulroney, former prime minister of Canada, said she remained extremely popular there.
He also told BBC Radio 5live that while her position might appear only "symbolic", she had played an important role on the world stage, notably in the fight against apartheid in South Africa.
"She has her own way of conveying a view on an important national or international issue that, if you know her in any way, is unmistakeably clear.
"The fact that she has exercised authority in such an impeccable manner, her own reputation and her own personal conduct has been flawless... the only conclusion I can come to is that she is a genuinely exceptional person."
Anti-monarchist group Republic said the Queen's long reign was a reason for reform not celebration.
Chief executive Graham Smith said: "It is now time for the country to look to the future and to choose a successor through free and fair elections, someone who can genuinely represent the nation."
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