Shortly after a video starring William and Kate was released on the official Kensington Royal Twitter account, Charles shared a tribute of his own, along with a poignant message.
The black-and-white image was taken at Buckingham Palace in 1914. Charles explained in his caption: "This is an image of St Patrick's Day from 1914, sent by an Army friend: Lord Roberts & Colonel Fitzclarence leading regimental proceedings for the Irish Guards. Both holders of the Victoria Cross, they were (as my friend points out) dead within the year."
WATCH: Prince William and Kate Middleton's special St Patrick's Day message
"What a wonderful picture," one follower remarked, to which he replied: "Fabulous, isn't it? Such pride."
Royal fans, meanwhile, were delighted to see William and Kate join forces for their St Patrick's Day video – which even saw the Prince speaking Irish!
Charles Spencer marked St Patrick's day with a historic photo
The Duke began his greeting saying, "Beannachtai na Feile Padraig oraibh," followed by Kate, who said: "We're delighted to wish you all a very Happy St Patrick's Day." William quipped to his wife: "How come you got that easy bit?"
The couple, both dressed in shades of green, then spoke about their own visit to Ireland last March, ahead of the first lockdown.
William, 38, said: "We were so thrilled to be able to visit Ireland just over a year ago, a few short weeks before all of our lives were turned upside down by the pandemic."
William and Kate after attending the 2019 Irish Guards St Patrick's Day Parade
Kate, 39, added: "The warm welcome that we received everywhere was a testament to the friendship between our two countries, and the strength of the relationship between the UK and Ireland."
The Cambridges appeared in the special video greeting alongside world leaders including US president Joe Biden, Canadian PM Justin Trudeau, New Zealand's leader Jacinda Ardern and Australian PM Scott Morrison.
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