Separated on a family vacation with their father Prince Charles while Diana was on various humanitarian trips, the two boys were scheduled to meet up with their mother again on August 31 – the day that she died at just 36 years old.
The two boys, who were 15 and 12 years old at the time of their mother’s death, spent time with Diana and her partner Dodi Fayed’s family earlier in the summer.
But when Diana and Dodi were killed, William and Harry were at Scotland’s Balmoral Castle, spending the remainder of the summer with their grandmother Queen Elizabeth II and their father Prince Charles.
Speaking on 2017 documentary Diana, Our Mother: Her Life and Legacy, Harry spoke to two landmine victims, who met with Diana on her very last humanitarian mission.
“You guys were almost the last people to see my mother,” Harry told Malic Bradaric and Zarko Peric in the documentary.
“Well, you saw my mother more recently than I did, I guess,” he said.
Following Prince Charles and Diana's split in 1992 and later their divorce in 1996, the two boys often had to split time between their two parents.
"There was the point where our parents split and... we never saw our mother enough or we never saw our father enough," Harry later added in the documentary.
"There was all that to contend with. And I don't pretend we're the only people to have to deal with that, but it was an interesting way of growing up."
In the documentary, Prince William and Prince Harry shared their greatest regret two decades on from their mother's death.
Looking back, the Duke of Cambridge remembers being in a rush to get off the phone to Diana so he could go back to playing with his cousins at Balmoral.
"Harry and I were in a desperate rush to say goodbye, you know, 'see you later'... if I'd known now obviously what was going to happen I wouldn't have been so blasé about it and everything else," Prince William said.
21 years on from her unexpected death, we remember Princess Diana's legacy.