The author and columnist enjoyed a sun-soaked break on Lake Como, Italy, with her hedge fund manager, James Matthews, after the pair attended the wedding of nutritional therapist Belle Porter and Max Robinson.
Pippa wowed in a red ruffled gown for the nuptials, and also showcased her effortless day-to-day style in a white lace top and pink shorts as she headed to a pool on the lake.
The mum-of-three also marked a new milestone for her young family this week, as eldest child, Arthur, four, is believed to have had his first day at school.
Pippa and James are also parents to two-year-old Grace and one-year-old Rose.
It's not known if Pippa will be throwing a party with family and friends, but she and James reside in a £15m property in Berkshire, close to Pippa's parents, Carole and Michael Middleton, and her younger brother, James Middleton and his wife, Alizee Thevenet.
However, Prince William and Kate are in France this weekend for the start of the Rugby World Cup.
Kate, who is patron of the Rugby Football Union, attended a group stage match between England and Argentina at the Stade de Marseille on Saturday night.
William, patron of the Welsh Rugby Union, will watch Wales compete against Fiji in another opening round match at the Stade de Bordeaux on Sunday.
No doubt the Middleton family will reunite at some stage to celebrate Pippa's milestone birthday.
In comparison, Kate, who is 20 months older than her younger sister, marked her 40th birthday in January 2022 quietly at her Norfolk home, Anmer Hall.
Kensington Palace also shared three stunning new portraits of the then Duchess of Cambridge to mark the occasion, which were taken by fashion photographer Paolo Roversi.
Kate looked beautiful in three Alexander McQueen gowns, including a red one-shoulder number.
During the photoshoot at Kew Gardens, Mr Roversi revealed how he persuaded Kate to dance an "accelerated waltz mixed with a pinch of rock 'n roll" after she donned an organza McQueen gown that reminded him of a "classical ballerina".