Kate Middleton, Prince William celebrate 13th wedding anniversary with never-before-seen photo

Prince William and Kate Middleton celebrated their 13th wedding anniversary on Monday with a special look inside their big day.

The Prince and Princess of Wales released a previously unseen picture from their wedding day in honor of the occasion.

"13 years ago today!" read the caption of the photo shared across their social media pages.

The picture was taken inside Buckingham Palace on April 29, 2011.

Happy anniversary to the Prince and Princess of Wales!

The couple's anniversary comes about a month after Kate announced in March that she was diagnosed with cancer and is undergoing chemotherapy.

SEE ALSO: Kate Middleton says she was diagnosed with cancer, is undergoing chemotherapy

Just one month earlier, in February, Buckingham Palace announced that Charles was diagnosed with cancer and had begun treatment.

The palace has not specified Charles' type of cancer, the stage of cancer or the type of treatment.

Likewise, Kensington Palace has not shared details on the type of cancer Kate was diagnosed with. She said in a video message that the cancer was discovered in post-operative tests after her planned abdominal surgery in mid-January.

ABC News contributed to this story.