Princess Anne greeted hundreds of Chelsea pensioners at the Royal Hospital Chelsea to celebrate Founder's Day, also known as Oak Apple Day. It commemorates King Charles II escaping from parliamentary forces after the Battle of Worcester in 1651 by hiding in an oak tree to avoid being captured. Chelsea pensioners wear oak leaves on the scarlet uniforms in recognition. A golden statue of Charles II is also decorated with oak leaves for the occasion.

Founders Day
Princess Anne, Princess Royal attends the Founder's Day Parade at The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images
Founders Day
A Chelsea penisoner sits on a mobility scooter during The Founder's Day Parade at The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images
Founders Day
Veterans badges are seen at the Founder's Day Parade at The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images

Founder's Day is held at the Royal Hospital Chelsea each year, on a date close to 29 May – Charles II's birthday. Inspired by Les Invalides in Paris, Charles II founded the Royal Hospital in 1682 as a retreat for veterans, with the idea that they could use it as a hostel instead of being paid pensions. The building was designed Sir Christopher Wren, whose other works include St Paul's Cathedral, the National Maritime Museum and Hampton Court Palace.

Founders Day
King Charles II of England (1630 - 1685) engraved by D Loggan, 1675Hulton Archive/ Getty Images
Founders Day
The fugitive King Charles II (1630 - 1685) and Major Carles hide in an oak tree in Boscobel Woods, Shropshire, in order to watch Oliver Cromwell's army approach. Meanwhile the king takes a nap with his head on Carles' lap, 1651Hulton Archive/ Getty Images
Founders Day
Princess Anne, Princess Royal attends the Founder's Day Parade at The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images
Founders Day
Princess Anne, Princess Royal attends the Founder's Day Parade at The Royal Hospital Chelsea in LondonEamonn M. McCormack/ Getty Images
Founders Day
The Founder's Day Parade at The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images
Founders Day
A Chelsea pensioner looks on during The Founder's Day Parade at The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images
Founders Day
Chelsea pensiors line up during the Founder's Day Parade at The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images
Founders Day
Princess Anne, Princess Royal attends the Founder's Day Parade at The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images
Founders Day
Princess Anne, Princess Royal attends the Founder's Day Parade at The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images
Founders Day
Princess Anne, Princess Royal attends the Founder's Day Parade at The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images
Founders Day
The Royal Hospital ChelseaEamonn M. McCormack/ Getty Images
Founders Day
The south front of The Royal Hospital Chelsea viewed from the River Thames, circa 1700Eamonn M. McCormack/ Getty Images
Founders Day
The Chelsea Pensioners giving three cheers to the Queen during Founder's day at the Royal Hospital in Chelsea, 1956Douglas Miller/ Getty Images
Founders Day
Founder's Day Parade at the Royal Hospital in London's Chelsea, Lord Hailsham takes a walk in the grounds with Colonel Braithewaite, the Governor of the hospital, 1933Fox Photos/ Getty Images
Founders Day
An oak leaf being pinned on Sargent Joseph Jones, of the 19th Hussars, the oldest Chelsea pensioner taking part in the Founder's Day Parade, 1956Eamonn M. McCormack/ Getty Images