Prince William and Kate Middleton's 13th Wedding Anniversary Feels 'Bittersweet' Due to Her Cancer Battle, Shares Royal Author

Kate Middleton and Prince William aren't going all out to celebrate their 13th wedding anniversary on Monday, April 29, as the couple and their loved ones are still dealing with the mother-of-three's cancer treatment.

"This is obviously the most bittersweet anniversary William and Kate have ever experienced," author Christopher Andersen spilled to a news outlet. "Their marriage is stronger than ever, they have three beautiful children – and now they are facing the greatest challenge of their married lives."

Adding to the stress is King Charles' own cancer battle.

"It’s hard to think of any marriage stronger than theirs or built on a more solid foundation. William and Kate are certainly more equipped emotionally and in every other way to handle whatever life throws at them – and with the dual diagnoses of Kate and the king, life has thrown them a lot," Andersen added.

Royal expert Ian Pelham Turner told the outlet the Prince and Princess of Wales are doing all they can to make sure the monarchy doesn't fall apart.

"The abdominal surgery, apart from being very painful, was emotionally draining for Kate," he revealed. "The couple had to come to terms with all of this even before the cancer announcement. William has stepped up to the plate, ensuring his family and his family are confidently moving forward."

Never miss a story — sign up for the OK! newsletter to stay up-to-date on the best of what OK! has to offer. It’s gossip too good to wait for!

"Together, [William and Kate] are working extremely hard to protect their children from any worries they may have and keep normality in their daily lives," the expert said. "For Prince Louis' 6th birthday, Kate kept her tradition going by baking midnight cakes for the young man. It’s a tradition she has done for many years."

On their anniversary, the couple uploaded a never-before-seen black and white photo from their special day in 2011, though former royal butler Grant Harrold believes they'll also do something at their home to mark the occasion.

"With everything going on with Kate, that won't be a public thing, that's going to be very private," Harrold explained. "Behind closed doors, I'm sure William will work to make sure she's spoiled."

"It's possible William might cook. He's quite a good cook, so it's possible he might do a little meal or something," he shared. "Again, it's just going to be a private affair between the two of them. Once again, there'll still be an exchange of gifts. I have no doubt they will give each other a gift, and I've no doubt they will exchange cards."

"I've no doubt that there'll be a special meal for them, whether it's a meal where they invite friends and family, or just them," Harrold stated. "Again, well-wishers will pass on their love with cards."

Fox News Digital spoke to the royal experts.