Punctuating a model month as a royal bride-to-be, Kate Middleton joined her fiancé for a special pre-Christmas lunch with Queen Elizabeth Thursday.
She and Prince William were driven the short hop from St. James's Palace to Buckingham Palace for the annual event that takes place before the Queen heads off for her seasonal holiday.
The newly engaged couple rode in a car with some protection officers that traveled behind Prince Charles and Camilla, the Duchess of Cornwall. Inside the palace, Kate, 28, mingled with a daunting array of royals, as all of the Queen's children were in attendance, along with most of her grandchildren. And while she was a guest at the 2008 wedding of the Queen's eldest grandchild, Peter Phillips, this is thought to be the first time Kate has had a lunch with the entire family at the palace.
Kate and William left about two and a half hours later – and this time, she was sandwiched between William and his brother Prince Harry in the back seat of an official SUV.
"It's a momentous day for her," royals author Judy Wade tells PEOPLE. "She must have met more than 40 members of the royal family and that must have been an ordeal for her to meet so many family members."
"When she left, Kate was looking supremely happy. She was smiling broadly," an observer says.
"She was beaming," adds another source watching at the palace gates.
As for the couple's holiday plans, William will be working on Christmas Day so he will not be with the Queen and family at Sandringham House in Norfolk, England. It remains to be seen if Kate will join him in North Wales or spend the day with her own family.