Earl Spencer marked the 29th anniversary of his sister Princess Diana's death on August 31, calling it "my least favourite day of the year." He shared three images of Althorp House, the Spencer ancestral home in Northamptonshire, on Instagram, with a flag flying at half-mast atop the estate.

Diana died in a car crash in Paris on August 31, 1997. The princess was survived by her two sons, Prince William and Prince Harry.

The somber Instagram post precedes the September 22 release of Spencer's memoir, "Swan Song." The book is expected to offer an intimate account of Diana's childhood, her marriage to the now-King Charles III, and the period following her death, which led to widespread public mourning. Spencer stated that the upcoming 30th anniversary of Diana's death prompted him to write his own memories to counter what he described as "untruths that have become accepted over time."

"Swan Song" is anticipated to detail the week of global grief and anger that followed Diana's death. Spencer, who was present during his sister's 15-year marriage, separation, and divorce from Charles, delivered a notable eulogy at her funeral. In his address, he emphasized Diana's ability to generate magic without a royal title and described her legacy as something that resonated with many women, allowing them to find personal meaning in her life experiences.

The book's cover features a black-and-white photograph of Diana. Publisher Daniel Bunyard described "Swan Song" as a work of "historic importance," highlighting Spencer's unique perspective as both a brother and a historian. Bunyard noted that the memoir is "extraordinarily moving, beautifully written and remarkably candid."

Diana and Charles Spencer grew up at Park House before the family moved to Althorp in 1975. The princess affectionately referred to her younger brother as 'Carlos,' while he called her 'Duch.'

Reports indicate that the Duke and Duchess of Sussex, along with their children Archie and Lilibet, visited Althorp last month and may have laid flowers on Diana's grave, which is located on an island in the estate's Oval Lake. The anniversary of Diana's death occurred shortly after the Duke and Duchess of Sussex relocated back to Britain after six years in the United States.