No footsie PJs allowed!

If Kate Middleton thought her royal wedding was stressful, she hasn't seen anything yet: In less than two weeks, the Duchess, 29, will spend her very first Christmas weekend with husband Prince William and the entire royal family at Sandringham manor, Queen Elizabeth's retreat in Norfolk, England.

Without her royal aides, parents or siblings at her side (no in-laws are invited), the princess will "quickly learn that this is not like any Christmas she has had," a royal source tells the new Us Weekly, out now.

Just packing for the holiday will be a challenge, as the fashion icon is expected to change up to five times a day for formal meals, black-tie- cocktails, church services and gift exchanges (novelty presents are encouraged!). To be sure, come the 25th, "Kate won't be able to spend the morning in her pajamas," the source says.

Adds Robert Jobson, author of William and Kate: The Love Story: "It will be a difficult baptism in protocol laced with pitfalls…She's first the first outsider since Camilla…so all eyes will be on her."

For all the details on the Duchess' very royal Christmas -- the outfits, the etiquette challenges, the quirky traditions and the Boxing Day pheasant shoot with Pippa! -- pick up the new Us Weekly, on stands Friday.