Today marked day one of Royal Ascot 2018, a five-day horse race attended by members of the royal family—including the Queen—every year. Though Duchess Meghan made her very first appearance at the Ascot today alongside Prince Harry, the Duchess of Cambridge was noticeably absent from the day's festivities.

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Anyone familiar with the traditions of the British royal family knows Kate Middleton's absence isn't all that surprising. The Duchess of Cambridge is still on maternity leave following the birth of Prince Louis, and can take off as much time as she wants for six months after the birth, royal biographer Duncan Larcombe told BAZAAR.com last month.

"The wedding doesn’t count because it was a 'family affair' but other than that, she will pick and choose her engagements," Larcombe said. This also explains why Duchess Kate attended the Trooping the Colour earlier this month.

As for Prince William, who also missed today's event? He was busy giving the keynote speech at the International Business Festival in Liverpool, of which he is a patron.

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Surprisingly, the Duchess of Cambridge has only attended the Royal Ascot twice since officially becoming a member of the royal family in 2011. Though Duchess Meghan attended her first Ascot a month after her own wedding, Duchess Kate waited a full five years. According to Hello!, this is because Kate and Prince William were busy raising their family. "William and Kate's focus was on his job and the children for a while, but they've stepped up their royal duties in the past couple of years," the publication's royal reporter, Emily Nash, said.

From left: The Duchess of Cambridge wears Dolce & Gabbana for her Royal Ascot debut in 2016; in 2017, she opted for Alexander McQueen. Getty Images

Thus, the Duchess of Cambridge's attended her first Royal Ascot in 2016, to mark the Queen's 90th birthday. She also attended in 2017, after Prince William left his job as an air ambulance helicopter pilot so the couple could put more focus on royal work.

See all the pictures from the 2018 Royal Ascot below.