The Queen is to begin her birthday celebrations with a lunch for 99 people who turn 80 on the same day as her.
The guests, who can bring one guest, will gather at Buckingham Palace before enjoying a three course meal in the grand Palace Ballroom.
Those attending were picked in a ballot earlier this year, after applying through their local Lords Lieutenant.
The Queen and her guests were all born on 21 April, 1926.
Invitations were issued on a pro-rata basis to Lords Lieutenant offices throughout Britain.
The birthday men and women will dine around 20 tables inside the royal home, with some seated next to the Queen or Duke of Edinburgh.
Guests will also be invited to a drink in the palace's Picture Gallery before lunch, where priceless portraits adorn the walls.
Betty Kay, of Doncaster, South Yorkshire, who is due to attend after her neice put her name forward as a surprise said she was "dumbfounded" at the invitation.
"It has always been a standing joke in the family that I've never been invited to the Queen's birthday party, what with us being born on the same day, never thinking it would actually come true," she said.
"Even when I was a youngster I would say, well you'd think she would invite me to her party."
Another guest, Edna Richards, will travel from Wolverhampton with her husband John.
"I am very proud I was chosen as I am a royalist.
"I think she is a marvel. I don't know how she does it all. Being the same age I know how she must feel."
She added: "I am not really nervous at the moment but I don't know how I will feel when I get there. It feels quite unreal."
The Queen will spend her 80th birthday at Windsor Castle, where Prince Charles will host a family dinner.
Other events are planned including a service of thanksgiving at St George's Chapel, Windsor on 23 April, and a service at St Paul's Cathedral on 15 June.
The monarch will also host a birthday Garden Party at Buckingham Palace for children from all over the UK.
On 17 June, the Queen will mark her official birthday with the Trooping the Colour.