The Queen is due to attend a huge street party on the Mall in London, the last event of the three-day celebration to mark her official 90th birthday.
Along with other senior royals she will join 10,000 people for the Patron’s Lunch – recognising her patronage of more than 600 organisations in the UK and around the Commonwealth.
The event has been organised by the Queen’s grandson Peter Phillips.
The festivities will also include a carnival parade.
Thousands of rain ponchos have been prepared for the open-air event in case of bad weather.
Security is expected to be tight at the event and guests, who have paid £150 each to attend, will go through body scanners when the site opens at 10:00 BST.
Ala Lloyd, who is the creative director of the carnival parade, said the eras of the Queen’s reign through the decades would be symbolised by different sections of the parade.
“We’ve got a lovely Commonwealth theme in the 50s with a giant Royal Yacht Britannia, flower power in the 60s, animals and nature in the 70s and crazy neon business going on in the 80s, and embarrassing shorts,” she said.
She said she was just trying to do something which is “affectionate and friendly”.
While the event takes place in London, smaller street parties are being held around the UK.
On Saturday, thousands of people turned out to watch the annual Trooping the Colour parade.
Dressed in a vivid lime green coat and matching hat, the Queen was escorted down the Mall in a horse-drawn carriage to the ceremony at Horse Guards Parade.
After the parade of more than 1,600 soldiers and 300 horses, she appeared on the balcony of Buckingham Palace with members of her family, including Princess Charlotte, for an RAF flypast.
BBC weather presenter Carol Kirkwood reporting from The Mall in London said rain was expected later, despite a clear sunny morning.
She said: “We will see some rain on The Mall when the Patron’s Lunch does start.
“Don’t take your brolly here if you’re coming to the Patron’s Lunch ponchos will be provided if it rains, and it looks very much that at some stage it’s going to.”
The rain is likely to be in short spells rather than a sustained shower, she added.