"It is the date of her accession, but it is also the date George VI – her father – died," Fitzwilliams said. "Essentially it's not a period where you would expect celebrations. Sunday is historic, of course, because that's when the Queen will be the first monarch to reach 70 years."
For the Queen, Fitzwilliams said, it has always been a twin anniversary — the day she became a monarch, and the day she lost her dad. "It was completely unexpected," he said, of the death of King George VI.
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"When usually she spends Christmas at Sandringham, this year she hasn't managed to do that because of the coronavirus restrictions but she is believed to be there and this is where she will obviously spend the day privately," he added.
"It may be the day that she became Queen, but it was also the day that her father died. She was very, very close to him," Hicks said. "You're probably dealing with the mixed emotions there."
Remembering her father on Sunday may be an opportunity the Queen takes to reflect on how his reign influenced her own. "It's a day where you'd reflect, your memories of the past, you think of loved ones no longer there," Fitzwilliams said.
"He was her inspiration because of his dedication to duty," Fitzwilliams said. King George VI succeeded to the throne unexpectedly, after his older brother Edward abdicated in 1936, just three years before Great Britain went to war.
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"This was a huge burden for him to bear," he added. "The Queen's sense of duty has been inspired by the example of her father."
Buckingham Palace did not immediately respond to Insider's request for comment.
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