Princess Catherine turned down Queen Elizabeth's rare Christmas offer and it shows she was always a stickler for tradition

The Prince and Princess of Wales have been playing a key part in modernising the monarchy in recent years, sometimes in ways you wouldn't necessarily think about. Nowadays we wouldn't bat an eyelash if we saw Catherine and William taking part in a selfie and it's quite normal for them to sign off messages simply 'C' or 'W'.

However, the Royal Family wouldn't be the same without tradition and the Princess showed early on in her relationship with Prince William that she was on board with this. Back in 2011, Princess Catherine is said to have turned down an unprecedented offer from Queen Elizabeth to join the family at Sandringham House for Christmas Day.

Typically partners don't spend the festive season with the royals until they're officially a member of the family and the Princess of Wales was William's girlfriend at the time.

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"By 2006, the couple had been dating seriously for the best part of five years," claimed royal author Robert Lacey Battle of Brothers. "Yet when William invited Kate to join him that year at Sandringham for the Royal Family's traditional Christmas lunch, she refused."

According to the writer, this was the "first time" Queen Elizabeth had ever "extended such an invitation to an unregistered 'girlfriend'". Whilst she no doubt greatly appreciated the monarch making a rare exception for her, Catherine reportedly "had her own take on that break with tradition" and it was that she'd spend Christmas Day in Norfolk "only when she was engaged".

She stuck to royal tradition and actually took things a step further in the end. Although the Prince and Princess of Wales announced their engagement in November 2010, Catherine's first Christmas at Sandringham wasn't until the following December after they were married.

Since then she's regularly celebrated the festive season there and the Princess wasn't the only one who stuck to this supposed rule. Mike Tindall also had his first royal Christmas in 2011 and he and Zara got married three months after the Waleses.

It might have been a source of comfort to Princess Catherine that she wasn't the only one experiencing this high-profile Christmas for the first time. In ITV's Our Queen At Ninety documentary, she confessed to feeling anxious over what to give her grandmother-in-law that momentous year.

"I can remember being at Sandringham, for the first time, at Christmas. And I was worried what to give the Queen as her Christmas present. I was thinking, 'Gosh, what should I give her?' I thought back to what I would give my own grandparents," she explained.

The Princess added, "And I thought, 'I'll make her something.' Which could have gone horribly wrong. But I decided to make my granny's recipe of chutney."

In a thoughtful move, the late Queen made sure it was on the table the next day and this didn't go unnoticed by Catherine. She said, "I think such a simple gesture went such a long way for me and I've noticed since she's done that on lots of occasions."

Spending Christmas with the monarch is probably a little nerve-wracking the first time so this was a lovely way to reassure the Princess.