It’s hard to believe the late Princess Diana would have been 56 years old on Saturday had she not been killed in a Paris car crash in August 1997. Instead, she lingers in memory as forever young, forever 36.

Her sons, Prince William and Prince Harry, plus William’s wife, Duchess Kate of Cambridge, will mark her birthday with a private service to rededicate her grave on her family’s estate, Althorp House, in Northamptonshire in the English countryside Saturday, just about two months before the 20th anniversary of her death.

Kensington Palace announced Wednesday in a brief statement that the service will be conducted by the Archbishop of Canterbury and will be attended by her family, the Spencers, including brother Charles, the Earl Spencer, and her two older sisters.

The palace said the service will be closed to the public and the media, and that the young royals asked for privacy.

Their mother was killed, along with her boyfriend at the time and a driver when the car she was in crashed into a pillar in a Paris traffic tunnel while being tailed by photographers. Only one person, a bodyguard, survived. Multiple inquiries later by the French and the British established that the crash happened because the driver was drunk, speeding and the victims were not wearing seat belts.

After 10 days of shock and mourning throughout Britain and a royal funeral at Westminster Abbey, Diana’s casket was taken 70 miles north of London to Althorp for burial. She was interred on a small island in a lake at the 509-year-old estate, set among 13,000 acres surrounding a walled 500-acre park.

Only Diana’s close family were there for the burial and her gravesite has been off-limits to the public ever since, although Althorp…