Charles Spencer is remembering his sister, Princess Diana.

In an Instagram post on Sunday, Aug. 31, Charles, 61, posted a photo of the flowers he brought to the burial site of the late princess of Wales at Althorp Estate as he marked the 28th anniversary of her death.

“Flowers we cut this morning from Althorp’s gardens for the Island,” he wrote in the caption, referring to Diana’s grave at his family’s estate in West Northamptonshire, England. “Always an impossible day,” Charles said.

The photo of the bouquet of white and pink roses in honor of Diana were shared alongside a snap of the private island where she is buried at Althorp Gardens.

Princess Diana was killed in a devastating car crash in Paris on Aug. 31, 1997, along with her partner Dodi Fayed and driver Henri Paul. The beloved royal left behind her two sons, Prince William, then 15, and Prince Harry, then 12, whom she shared with King Charles, formerly the Prince of Wales.

Princess Diana’s final resting place at Althorp Estate was decided by her brother Charles. She is buried on a secluded private island in Pleasure Gardens on the grounds, which are also known as Oval Lake Grave.

Charles previously told PEOPLE that he wished for his sister to be buried in a “secure place” where her loved ones could visit her privately, particularly her sons William, 43, and Harry, 40, without public disruption.

“I think it’s very important for them to be there with her,” he said. “It is, luckily, very tranquil here and they can come and go as they wish whenever they want. And that’s very lovely for me to know that.”

Charles spoke of his decision in his 1998 book, Althorp: The Story of an English House, “We all agreed that, with its beauty and tranquility, this was the place for Diana to be.”

While Diana’s grave on the island isn’t accessible to the public, those who seek to honor the late princess can pay tribute to her at a separate memorial built at the grounds, built in July 1998, per the Washington Post.