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Large crowds are expected to mark her milestone and to enjoy the procession, featuring more than 1500 soldiers and officers and several hundred horses.
Later, the royal couple will lead the family onto the palace balcony to wave to the throngs as Royal Air Force jets and some vintage World War II planes fly overhead.
Many observers expect 1-year-old Princess Charlotte to make her first balcony appearance with her parents, Prince William and his wife Kate.
The festivities continue on Sunday with a massive street party in front of the palace. The palace has invited roughly 10 000 people who work at charities supported by the queen to the open air festivities.
Street parties will also be held in many towns and cities throughout Britain and in other Commonwealth countries.
The queen’s real birthday is in April. The official birthday is timed in hope of fair weather — a custom that began in the 18th century.
