Politicians and members of the Royal Family are due to attend a national service of thanksgiving to celebrate the Queen's official 80th birthday.
The Queen marked her actual 80th birthday with a family dinner
Those expected at the service, at St Paul's Cathedral in London, include Prince Charles, sons William and Harry, and Prime Minister Tony Blair.
For her actual birthday in April, she enjoyed a family dinner at Windsor.
On Saturday - her official birthday - she will attend the Trooping the Colour birthday parade.
The royal tradition of two birthdays dates back to the time when, if monarchs were born in winter, the weather was deemed unsuitable for outdoor events.
Although the Prince of Wales is among the Royals expected at Thursday's service, the Duchess of Cornwall will not attend because of the recent death of her father.
The children of the Queen's late sister Princess Margaret will also be in attendance.
A two-day sale of their mother's possessions ended on Wednesday night, raising £13.5m.
BIRTHDAY LUNCH MENU
Smoked salmon with blinis, woodland sorrel and wild cress
Pan-fried turbot with cockles and oxtail
Loin of roe venison with potato cake, roast roots, creamed cabbage and game gravy
Custard tart with nutmeg
Invited politicians include Mr Blair, Chancellor Gordon Brown, Conservative leader David Cameron and Lib Dem leader Sir Menzies Campbell.
After Thursday's service, the Queen will attend a lunch in her honour at Mansion House attended by more than 350 guests.
Four top British chefs, chosen by viewers of the BBC television programme Great British Menu, will prepare the starter, fish course, main course and pudding.
Richard Corrigan, Bryn Williams, Nick Nairn and Marcus Wareing will all cook courses.
Mr Nairn, who is cooking the main course, told BBC News last minute preparations were making him nervous.
"The caterers who are doing the actual coal face work have phoned up saying that they've lost 40 portions of venison. But these things do tend to happen in these big events and I'm sure that it will all resolve itself satisfactorily," he said.
"But at the moment, nerves are beginning to settle in."
On Thursday evening, the attention will shift to the Queen's husband.
The massed bands of the Royal Marines will perform on Horse Guards to mark the Duke of Edinburgh's 85th birthday.
On Saturday, Phillip celebrated his birthday with a private ceremony at Windsor Castle.
A royal gun salute was fired at Hyde Park and the Tower of London, and the union jack was flown on government buildings.
The royal couple will celebrate their 60th wedding anniversary next year.