The couple, who met more than eight years ago while attending the University of St. Andrews in Scotland, got engaged last month during a vacation in Kenya. William proposed with the same engagement ring that belonged to his mother — an oval blue 18-carat sapphire surrounded by 14 smaller white diamonds, according to Us.
"2011 would be Diana's 50th birthday and also the 30th anniversary of her wedding to his father," William and Harry: Behind the Palace Walls author Katie Nicholl told Us. "This will be William's way to make sure his mother's memory won't be forgotten on the most important day of his life."
Diana and Prince Charles wed on July 29, 1981. The televised event attracted more than 750 million viewers.
The 28-year-old prince is second in line to the throne after his father, Prince Charles. Middleton, also 28, is the daughter of self-made millionaires and works for the family mail-order business, which specializes in children's parties.
The couple will live in north Wales following their marriage, where Prince William will continue to serve with the Royal Air Force, according to royal officials.
William's grandparents, Queen Elizabeth II and Prince Philip, "are absolutely delighted for them both," Buckingham Palace said.