Kate Middleton, Prince William
Kate Middleton and Prince William both threw a fancy dress party when they turned 21. Pictured: Middleton, Prince William greet well wishers as she attend Christmas Day Church service at Church of St Mary Magdalene on the Sandringham estate on December 25, 2018 in King's Lynn, England. Getty Images/Stephen Pond

Kate Middleton and Prince William love to attend parties, but there is one in particular that is believed to be their favorite.

In the book “Kate: A Biography,” author Marcia Moody said that the Duke and Duchess of Cambridge both loved fancy dress parties. In the summer of 2003, they both attended each other’s 21st birthday parties.

“Both Kate and William opted for a fancy-dress theme for their parties. Kate chose the elegant and dramatic flair of the 1920s and wore a stunning flapper dress for the champagne reception at her parents’ home in June,” she said.

According to Moody, Prince William slipped into the party after the other guests have arrived. The second in line to the throne’s 21st birthday bash was held at Windsor Castle, and he chose an “Out of Africa” theme. Prince William’s guests arrived at the venue in their costumes.

Queen Elizabeth II also joined in the fun, and she dressed as the Queen of Swaziland. Her Majesty stunned in her glittery white gown and African headdress.

“The whole venue was decorated to look like the African jungle, with huge model elephants the corners tribal masks on the walls and a lifelike replica giraffe’s head above the bar, while a band called Shakiramba were flown over from Botswana, and William took to the stage to play drums,” Moody said.

In related news, Prince William and Middleton also made headlines this week after it was reported the couple’s fondness for partying is also what ended their relationship years ago. Jessica Jayne, the author of “The Duchess of Cambridge: How Kate Middleton Became A Future Queen,” said that Middleton felt humiliated by Prince William’s partying habits that she decided to split from him.

Middleton also found out that Prince William flirted and danced with a girl while in club Boujis instead of seeing her. A photo of Prince William with two girls at a party was also published in the tabloids. Upon seeing the photos, Middleton decided to call it quits.

But the parents of Prince George, Princess Charlotte, and Prince Louis got back together months later.