Clue that will expose royal feud
They haven't been seen together in public for months, so when the young royal in-laws, who are supposedly at war, are pictured together at Christmas, the eyes of the world will be on them.
Meghan Markle and Kate Middleton had an afternoon in the sun at Wimbledon in July, but they have not been seen in close company since.
They arrived and left Princess Eugenie's wedding in October separately, and did the same again yesterday at the Queen's annual Christmas lunch.
But on Christmas Day royal fans will be lining the streets of Sandringham, Norfolk, in the hope of a glimpse of Meghan and Kate, along with her husbands, brothers Prince Harry and Prince William.
And everyone from body language specialists to royal experts will be waiting to see what happens.
But while what happens immediately after the church service will be important, something far more significant will occur away from prying eyes later that night.
Veteran royal watcher Phil Dampier, author of Royally Suited: Harry And Meghan In Their Own Words, told news.com.au he expected there would be happy scenes in public.
"I'm sure the fab four will be all smiles as they walk to church and put on a show of unity, but royal watchers will be studying the body language."
But where Meghan and Harry actually spend that night will provide the biggest clue as to what is really going on.
The couple spent last year with William and Kate but have reportedly turned down an offer to stay with them again this year at their home Anmer Hall, on the Queen's Sandringham Estate.
Dampier said that would give the biggest hint.
"The key is whether Harry and Meghan stay at Anmer Hall or the big house. If it's the latter, I think we can assume there is still a problem."
A royal source told The Sun that would be an unusual move, as there was far less pace there.
"There's not a huge amount of room at Sandringham so it's quite surprising. Maybe they just want their own space."
The source added: "Things are civil between the couples but they don't spend much time together."
Dampier hoped the tension, if it was there, didn't last long and impact on their lives going forward.
"I hope and think the four of them are mature enough to patch things up. If Meghan is not getting on with her dad and her new family, it's not a good sign."
After the church service and a turkey lunch, the family usually gathers to watch the Queen's message on TV. Board games and movies often followed.
If Meghan and Harry arrived late on Christmas Eve and left early on Boxing Day, and Harry did in fact miss the pheasant shoot, they would only have been with the family a day-and-a-half.
All these events meant it would be "difficult to hide" if there were "real problems", Dampier said in a newspaper column this week.
The fab four, as they have been dubbed, delighted crowds last year when they walked side-by-side out of the church and beamed at wellwishers. Back then the excitement was already building towards Meghan and Harry's lavish wedding at Windsor Castle and the introduction of a glamorous American actor to the royal family.
A year on, Meghan remains hugely popular and her recent tour of Australia and New Zealand proved her superstar appeal. She is popular in the UK as well, but stories of a feud between the Sussexes and the Cambridges persist, as do claims she is difficult to work with.
She has also been accused of driving a wedge between Harry and William. None of the rumours have been confirmed - there is rarely any comment from palace officials - apart from a Buckingham Palace spokesman who said, "This never happened," when asked about Meghan and Kate having words over Meghan's alleged rudeness to staff.
Royal biographer Christopher Wilson told the Daily Mirror: "Sandringham Christmases can be an ordeal, even for those born royal - and generally those who've married into the family can't wait to get away."
Princess Diana's former butler Paul Burrell told the Mirror this week Meghan should brace for an intense period full of "the biggest personalities and egos".
Mr Burrell often comments about his time with the royals - for a fee. In his latest interview, he said Diana's background helped prepare her for Sandringham.
"It's what I said to Diana when she first came into the family; try to get airtime with the Queen, speak into her ear," he said.
"Nothing will happen if you're close to the Queen's grandson. But if you stray away from him (Harry) and start to do things by yourself, that's when there could be problems."
Even Meghan's time in the cutthroat acting world with demanding filming schedules would prepare her for what she was about to experience this Christmas.
"Imagine Downton, but ramp it up. Sandringham is Downton Abbey on speed."