Catherine, the Princess of Wales, wished the Irish Guards good luck on their final rehearsal before the Trooping of the Color parade in honor of the king's birthday.

CBS News previously confirmed that Kate would not be attending the Colonel's Review, the formal dress rehearsal for the parade held on Saturday.

In a letter shared by the Irish Guards on X, the Princess of Wales apologized for not being there in her capacity as the guards' honorary colonel.

"Being your colonel remains a great honor and I am very sorry that I'm unable to take the salute at this year's Colonel's Review," she wrote in the letter. "Please pass my apologies to the whole regiment, however I do hope that I am able to represent you all once again very soon."

The review is a dress rehearsal for the annual military parade held each June to mark the monarch's official birthday. King Charles III will oversee the ceremony on June 15.

It's still unclear if Kate will attend the ceremony next week.

Kate is currently undergoing treatment after she was diagnosed with cancer earlier this year. She stepped away from her public duties and is not expected to return to work until she is cleared by her medical team. Kensington Palace has not given any indication of when that might be. She has revealed few details about her illness or treatment.

Her husband, Prince William, said last month that Kate was "doing well."

The king, who is also undergoing his own cancer treatment, has eased back into public duties. He attended commemorations this week for the 80th anniversary of D-Day, the invasion of Nazi-occupied Europe on June 6, 1944.

Trooping the Color is a 260-year-old tradition in which troops in full dress uniform parade past the king with their ceremonial flag, also known as their "color."

Charles is likely to travel to the event by carriage with Queen Camilla and is expected to watch the ceremony seated on a dais, rather than on horseback as he did last year.

The Irish Guards said they were touched by the letter from Kate and wished her well in her recovery.

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• Catherine Princess of Wales