Along with other senior royals she will join 10,000 people for the Patron's Lunch.
The event has been organised by the Queen's grandson Peter Phillips and guests have paid 150 pounds each to attend.
The festivities will also include a carnival parade as thousands of rain ponchos have been prepared for the open-air event in case of bad weather.
Ala Lloyd, who is the creative director of the carnival parade, told the BBC the eras of the Queen's reign through the decades are being symbolised by different sections of the parade.
While the event takes place in London, smaller street parties are being held around the UK.
Yesterday, thousands of people turned out to watch the annual Trooping the Colour parade, where Princess Charlotte stole the show with her 90-year-old great grandmother dressed in a lime green coat and matching hat emerged at the Buckingham Palace balcony for an RAF flypast.
