Kate Middleton and Prince William Sport Poppy Pins at Festival of Remembrance

CHRIS RADBURN/POOL/AFP via Getty Kate Middleton and Prince William

Kate Middleton and Prince William took part in an annual tradition honoring the U.K.'s fallen soldiers ahead of Remembrance Sunday.

The Prince and Princess of Wales, both 40, joined fellow members of the royal family on Saturday at Royal Albert Hall in London for the Festival of Remembrance, a yearly musical tribute to remember those from Britain and the Commonwealth who died in war.

The Festival of Remembrance was also attended by King Charles and Queen Camilla; Prince Edward and Sophie, Countess of Wessex; Princess Anne and her husband, Vice Admiral Sir Tim Laurence; The Duke and Duchess of Gloucester; The Duke of Kent and Princess Alexandra.

RELATED: Kate Middleton Surprises 3-Year-Old Boy with a Special Gift: Her Own Poppy Pin!

The men wore suits while the women dressed in black. All adorned their outfits with red poppy pins, the symbol that has been used since 1921 to commemorate military members who have died in war.

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The poppy symbol is believed to have come from the poem "In Flanders Fields" by John McCrae, a poem about World War I, which reads in part:

In Flanders fields the poppies blow
Between the crosses, row on row,
That mark our place; and in the sky
The larks, still bravely singing, fly
Scarce heard amid the guns below.

In the U.K., the pins are sold by the Royal British Legion to help raise money for veterans.

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Kate opted to wear three poppies on her otherwise black ensemble. The symbolic flowers are believed to honor her great-grandmother Olive Middleton's three brothers, who all fought and died in World War 1.

Earlier this week, Kate shared a special moment with a 3-year-old boy named Akeem during her visit to Colham Manor Children's Centre. After Akeem showed interest in her poppy pin, the Princess of Wales took it off of her jacket and gave it to him while sharing why she was wearing the accessory.

"Do you know what this is for?" Kate asked. "It's for remembering all the soldiers who died in the war. There you go — that's for you."

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On Sunday, members of the royal family will gather again for the National Service of Remembrance, also known as Remembrance Sunday, a ceremony at The Cenotaph war memorial in London.

This year will mark the first Remembrance Sunday since Queen Elizabeth's death and King Charles' accession to the throne. The historic Queen died on Sept. 8 at age 96 of old age, according to a document published by National Records of Scotland.