Life is full of milestone birthdays worth a big celebration, but turning 21 marks the start of adulthood.
So, it's no surprise that Carole and Michael Middleton pulled out all the stops to mark the occasion for their eldest daughter Kate.
Six months after her actual birthday, in June 2003, they organised a big 1920s themed party and sit-down dinner for Kate's closest friends and family, including her new secret boyfriend Prince William.
But the special day was almost ruined by one of Kate's friends Virginia.
According to royal expert Katie Nicholl, Virginia, also known as Ginnie by friends, decided to host her 21st party on the same day as Kate’s.
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Katie spoke to some of the couple’s friends about the party for her book, Kate: Our Future Queen, who said: "It caused a real divide and a bit of social rift. Kate was very upset."
They added that Kate "cut Ginny off a bit after that".
Even though Kate had sent the invites for her party out first, many of her friends, including William, found themselves in the awkward position of having to decide which party to attend.
While a few people decided to go to Ginnie's party over Kate's, William kept his promise to turn up, albeit slightly late.
When he pulled up in his Volkswagen Golf, Kate slipped out of the cocktail reception to greet him.
It was during the evening's festivities that he met her parents for the first time.
He also made quite the impression on Kate's friends, who he mingled with effortlessly.
One of the guests who were there on the evening told Katie: "Lots of [the guests] knew each other, and William knew quite a few people there, which made it very relaxed.
“We all gave him his privacy, and he kept himself to himself.
“It was a sit-down dinner and dance and William looked very dapper; he seemed to be having a lot of fun - we all were.”
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