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9 July 2015 Last updated at 09:20 BST
The Patron's Lunch will be attended by 10,000 guests - many from charities supported by the Queen - in London in June next year.
The Duke of Edinburgh, Prince William and Prince Harry will be at the party too.
Organiser Peter Phillips - the Queen's grandson - said his grandmother was "excited" about the event.
The street party will be the finale to a weekend of national events to celebrate the Queen's official 90th birthday.
A giant street party will be held next year to celebrate the Queen's 90th birthday.