This is the reason why Kate Middleton declined Queen Elizabeth’s invitation to spend Christmas in Sandringham in 2006, breaking with tradition
Royal biographer Robert Lacey described asking a lover to attend the customary royal Christmas celebration as “the first time the Queen had extended such an invitation.”

Queen Elizabeth defied convention in 2006 by inviting Kate Middleton, who was then Prince William’s girlfriend, to Sandringham for Christmas.
According to accepted custom, royal companions were expected to follow the “no ring, no bring” rule, which prohibited them from attending formal royal family events such as the customary Christmas at Sandringham until they were married.

Queen Elizabeth’s invitation was significant because William and Kate weren’t even engaged yet and wouldn’t be for another four years, until 2010. However, Kate, who was 24 at the time, graciously declined the Queen.
According to The Daily Mail, the Queen brought Kate to Sandringham for the first time with a girlfriend in 2006. Kate “politely declined the offer, opting to stick with tradition and wait until she could attend as a married woman — with the ring to prove it,” the outlet claimed, in a surprising turn of events.

It was proof that Kate “valued the importance of tradition and patience,” according to The Daily Mail.
Royal writer Robert Lacey stated in his 2020 book Battle of Brothers: William and Harry — The Inside Story of a Family in Tumult that the couple had been dating seriously for over five years by 2006. However, when William asked Kate to attend the royal family’s customary Christmas celebration at Sandringham that year,