The Princess Royal is confirmed to host an historic event at Buckingham Palace in May.
The King's loyal sister, 73, will be the hostess with the mostest as she holds a garden party for the Not Forgotten Association at the London residence on 17 May.
The charity supports military veterans and wounded serving personnel through a varied programme of social activities and a garden party has been held at Buckingham Palace in support since 1921.
Princess Anne took over the patronage from the Duchess of Kent in 2000, but the charity's first ever royal patron was Mary, Princess Royal and Countess of Harewood.
While the Princess Royal often plays host at the Not Forgotten Association garden party, other royals who have stepped in are the Duchess of Edinburgh, Prince Harry, Princess Beatrice and Princess Eugenie.
Garden parties will also take place at the palace on 8 and 21 May with royal attendance still to be confirmed.
It's uncertain how many large-scale public events the King will be able to attend in the coming months given cancer diagnosis, but a palace source said after the Easter Sunday service that Charles has "responded to treatment very encouragingly" and the "road ahead is looking very positive".
In the coming months, the royal family's spring and summer calendar includes annual events such as the Chelsea Flower Show, the Buckingham Palace Garden Parties, Trooping the Colour and Royal Ascot.
Speaking about how the likes of the Princess Royal and the Duke and Duchess of Edinburgh are continuing to support the King by increasing their engagements, royal author Robert Hardman tells HELLO!'s A Right Royal Podcast: "Garden parties are a huge part of the royal machine. At each one, 8,000 people are invited to tea and that's well in hand now, those invitations have gone out, people are buying their hats and their new suits, and it's going to be absolutely a day in the lifetime and no one wants to let them down and rearrange those things so that will carry on."
The King was joined in Windsor on Easter Sunday by his siblings and their respective spouses – Princess Anne and Vice Admiral Sir Tim Laurence, the Duke and Duchess of Edinburgh and their son, James, Earl of Wessex, and the Duke of York.
Sarah, Duchess of York joined her ex-husband, Prince Andrew, at the service for the first time since 1991. The pair divorced in 1996 but have remained on amicable terms and even live together at Royal Lodge in Windsor.