Tomorrow will be a significant day in the Princess of Wales' recovery while undergoing treatment following her cancer diagnosis. She'll be making her first public appearance in months at the King's Birthday Parade, also known as Trooping the Color. Along with a new picture, Kate posted a personal message this evening saying she's making good progress, but adds that as anyone going through chemotherapy will know, there are good days and bad days. She also warns she's not out of the woods yet. Johnny Diamond has our top story.

A new picture of the princess from the palace to mark a new stage in her journey. It's been almost three months since the princess revealed that she was undergoing chemotherapy, something that followed surgery earlier in the year.

The surgery was successful. However, tests after the operation found cancer had been present. My medical team therefore advised that I should undergo a course of preventative chemotherapy, and I'm now in the early stages of that treatment.

Since then, Prince William has worked alone, and the palace has remained silent. Though last week, when asked about Kate's progress, the She said she was getting better. She'd have loved to have been at the D-day Commemoration, he said. She'd love to be here today. From the Princess this evening, a message about these past long months. I'm making good Progress, she wrote. But as anyone going through chemotherapy will know, there are good days and bad days. On those bad days, you feel weak, tired, and you have to give in to your body resting. But on the good days, when you feel stronger, you want to make the most of feeling well. My treatment, she wrote, is ongoing and will be for a few more months. Tomorrow is the King's birthday parade, Trooping the Color, one of the biggest days of the Royal Year, and the Princess says she will be there, back in the public eye with her children. There may be more public engagements to come this summer, but each one will depend on the princess's his health. I am, the Princess wrote today, not out of the woods yet, but from Windsor comes a message and a picture that speak of hope for days to come.

Johnny Diamond, BBC News.

Well, Helena Wilkinson is in Windsor live for us tonight. Helena, the Troopy in the Color ceremony tomorrow, taking on an added significance now for the Royal family.

Yes, absolutely. And that photograph, Clive, that you saw in Johnny's report of Catherine was taken here near the family home in Windsor earlier this week. What is striking is just how personal that statement that we heard is from Catherine. She talks about how she feels on the good and bad days, and I think that will resonate with people who have been affected by cancer. She has been missed, very much missed from public life, and so the news tomorrow that she will be attending Trooping the Color is positive news. Her father-in-law, the King, King Charles, has said this evening that he is delighted that Catherine will be able to attend tomorrow. After tomorrow, we might see Catherine at some public engagements over the summer months, but it all depends on how she feels. She is still undergoing treatment. So very much the focus going forward, Clive, still is on her recovery.