The Duchess and Duke of Cambridge
had better have some surprises up his sleeve for Christmas Day
at his wife Kate's
Father-in-law Michael Middleton likes to pull a few party tricks – including dressing up in costume, his daughter Pippa
"My father has developed this funny tradition of surprising us at some point by appearing in fancy dress," the Celebrate: A Year of Festivities for Families and Friends"
author, 29, writes in the latest holiday edition of the political magazine The Spectator
"He buys a new costume each year and typically gets a bit carried away – a couple of years ago, he appeared in an inflatable sumo outfit."
She calls the tradition "endearing, and only quite embarrassing."
The Middleton home in Bucklebury, Berkshire
, will provide a peaceful, rural haven for the royal couple, especially as Kate continues to recover from the severe morning sickness
in the first weeks of her pregnancy
"The Middletons' Christmas should be blissfully calm," says her sister. "We're good at keeping each other's spirits up."