Sarah Ferguson shared a brief, emotional tribute to the late Princess Diana this week, on what would have been Diana’s 63rd birthday.

In an Instagram post Monday, Sarah, the Duchess of York, shared a sweet message about Diana.

“Happy birthday to my dear friend, Diana,” she wrote in the caption. “You were a pillar of light and love. And what a legacy you have left behind. I will forever remember our laughter and the kindred, kind spirit I found in you. I am sure you are watching over us always. Rest in peace my friend.”

Her message accompanied a photo of the two side-by-side on the balcony of Buckingham Palace in 1990.

Sarah was not only Diana's sister-in-law, as Diana was married to King Charles III (at the time, Prince of Wales) and Sarah to younger brother Prince Andrew, but counted Diana as one of her best friends from girlhood, and actually the person who introduced her to Andrew, whom she wed in 1986 and divorced a decade later, but remains close with.

“We were best friends from when she was 14 and I was 15,” Fergie told People in 2021. “She taught me so much of public life. She was so brave. We used to have the most incredible time together.”

“We promised each other we would always be together—there was never any daylight between us,” she continued. “But everybody wanted that, because we were so strong together. People want to break something so strong.”

Only a year apart in age and close friends, the media was fascinated with Fergie and Diana. In 1998, Vanity Fair mentioned that the two had even dressed up as policewomen and tried to crash Prince Andrew’s stag party years before in a lighthearted prank.

Fergie has continued to speak highly of her friend, who died in 1997. Last year, when asked about Prince Harry and Meghan Markle’s choice to step down from their roles as working royals in 2020, she kept things positive.

“I don't believe that any single person has the right to judge another person. I'm not in a position myself to make any judgments. I have been judged all my life, and I have no judgment on the Sussexes,” she said. “I think that Diana would be incredibly proud of her grandchildren. And not just the Sussex grandchildren, but Will's children.”