The Princess of Wales sent a letter to the Irish Guards earlier this week -- which they posted to X -- applauding them for their vigorous training and preparation for the Colonel's Review ... a last-minute rehearsal for a much larger parade.

Middleton -- who serves as the regiment's colonel ceremoniously, and would usually attend and salute the men before an event called Trooping the Colour -- then apologizes for not being able to come this year. While she doesn't give a reason, it's pretty clear her health's keeping her away.

The Irish Guard -- a foot guard regiment serving as both protectors of the Royal Family and in active combat roles -- will take part in Trooping the Colour ... a large parade held annually honoring the official birthday of the monarch.

It's unclear if KM will appear at the actual event -- again, this was just one rehearsal -- set for next Saturday. Kensington Palace has not yet made a statement regarding the princess' attendance, although it has been reported that she won't be coming.

Princess Kate's stayed out of the public eye since revealing she had cancer in a shocking X post in March ... and, it would come as a surprise to many if she did pop up at Trooping the Colour.

Prince William attended a celebration in Normandy to mark the 80th anniversary of D-Day earlier this week sans Kate. He told a veteran who asked about her she's feeling much better -- but, it seems she's not up to public engagements quite yet.

There's still no timetable for her return to public duties ... and, we'll have to wait for Kensington Palace's next announcement.