Kate Middleton reunites with Queen Letizia and Queen Maxima at Garter Day

Queen Elizabeth, Prince William, Prince Charles and other royals were out and about in their most fancy ceremonial dress at Windsor Castle on Monday.

Yet despite all the pomp and pageantry on display at the annual Garter Day ceremony, it was the royal reunion happening on the sidelines that really claimed the limelight.

The Duchess of Cambridge met again with Queen Letizia of Spain and Queen Maxima of the Netherlands, who were in Windsor for a very special occasion.

Letizia and Maxima's husbands were both invested into the prestigious Order of the Garter, an order of knights dating back almost seven decades.

During the day's formal proceedings, they were joined by Catherine, along with Camilla, Duchess of Cornwall and Sophie, Countess of Wessex.

Kate, wearing a white Catherine Walker coat with black trimming, was seen chatting with the visiting queens as they stood on the Galilee Porch during the procession through the castle grounds.

The Duchess and husband Prince William then departed the event in a carriage with the visiting King and Queen of Spain.

The Order of the Garter is the oldest and most senior Order of Chivalry in Britain, having been created by King Edward III almost 700 years ago.

The current Order comprises several senior royals, including the Queen, Prince Charles, Princess Anne, Prince Andrew, Prince Edward and the Duke of Cambridge.

The Order also includes Knights personally selected by Her Majesty.

These knights, both male and female, are chosen in recognition of their public service and come from a variety of backgrounds, not necessarily aristocratic.

This year, both King Willem-Alexander of the Netherlands and King Felipe were invested as Supernumerary (i.e. 'Stranger') Knights of the Garter.

Garter Day is one of the more traditional events on Queen Elizabeth's calendar, as evidenced by the striking ceremonial dress on display.

The day begins with the monarch investing any new Companions of the Order in the throne room at Windsor Castle, before a lunch is hosted for the members and officers.

Then, wearing velvet robes and plumed hats, the Queen and her Order of the Garter undertake a procession through Windsor Castle on foot, accompanied by a marching band.

This is followed by an installation ceremony for new Companions of the Order inside St George's Chapel.