Kate Middleton Throwing Prince William Small Dinner for 30th Birthday

Kate Middleton and Prince William
Kate Middleton and Prince William on May 18, 2012 in Windsor, England.  Arthur Edwards - WPA Pool/Getty

The Duchess will be the hostess with mostest on Thursday evening — throwing an intimate dinner bash in honor of hubby Prince William's 30th birthday.

"Kate [Middleton] had been planning a small dinner party with their friends and some family," a source tells Us Weekly of the low-key soiree, to be held at Prince Charles' countryside house.

Despite the major milestone — and his $15.5 million windfall from late mother Princess Diana's estate — the Duke of Cambridge hasn't been idle this week. "William is working as usual during his birthday week," adds a palace aide, explaining that William has been training at the Royal Air Force base in Gloucestershire. "Tonight he will have a private celebration with family and friends." (The dinner bash was originally going to take place at the couple's Anglesey, Wales home, but William's last-minute training assignment moved the festivities to Highgrove, located near Gloucestershire.)

The super-casual party doesn't surprise those closest to William and Kate, also 30. When the Royal Air Force Search and Rescue pilot comes home from work, he likes to make "sweet gestures" for his wife of one year, another source says. He'll typically pick up a treat for Kate en route to their home in Anglesy, Wales — or even have flowers delivered.

In turn, "Kate will cook his favorite dinners after he gets home from work . . . or she'll order movies on Amazon for them to watch on the couch."

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