Before there was Kate’s Alexander McQueen wedding gown, there was Princess Diana’s – and it was next-level famous. Designed by David and Elizabeth Emanuel, the frothy silk and taffeta number boasted a 25-foot train and 10,000 pearls. The dress has spent 17 years in the hands of her brother, Charles Spencer, but come September 15, it will go to her sons.

As outlined in Diana’s will, the ivory gown must be given to Princes William and Harry following Harry’s 30th birthday. With that milestone just a couple weeks away, fans are speculating the fate of the dress. PEOPLE guesses it might go on display in London’s royal palaces, which isn’t far-fetched considering it spent a decade as the centerpiece of the museum at Althorp, Diana’s family’s Northampton estate.

By Susie Kostaras, Associate Editor

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