It is not known what the big event is, but it is important enough for the royals to want to mark it during their special weekend.
The Duke and Duchess were married before the eyes of the world on April 29 last year, and their first wedding anniversary falls tomorrow.
Since they walked down the aisle of Westminster Abbey as man and wife they have had an eventful 12 months.
The royal couple honeymooned in the Seychelles, before touring Canada and California last summer then returning home to the UK to settle down to married life.
William has recently returned from a six-week deployment in the Falkland Islands as an RAF search and rescue helicopter co-pilot, while Kate has carried out a string of solo engagements around the country.
It is thought the royals will travel back to their home in North Wales on Sunday to celebrate their anniversary privately.
Behind closed doors the couple are likely to prepare a romantic meal to mark 12 months as husband and wife.
The first wedding anniversary is traditionally known as paper and the Duke and Duchess may exchange gifts.
William could give Kate paper tickets for a music concert and she may treat him to the same - the Duke is a fan of R&B grooves and rap, while the Duchess has developed an interest in classical music.
A St James's Palace spokesman would not comment on the couple's plans for the anniversary, saying only: "They will spend the weekend with friends who have a celebration."