The Queen is marking 70 years on the throne, but there's a good reason why she won't celebrate the anniversary

• Queen Elizabeth II will be the first monarch in history to serve 70 years on the throne.
• She became Queen of England on February 6, 1952, at the age of 21 while on tour in Kenya.

Sunday marks the Queen's 70th anniversary on the throne, but unlike the celebrations and revelry planned to honor her Platinum Jubilee in June, she will spend the day paying homage to her father and predecessor, King George VI.

Royal expert Richard Fitzwilliams told Insider the Queen is reportedly set to spend the anniversary privately at her home in Sandringham.

According to the Royal website, a new sovereign succeeds to the throne "as soon as his or her predecessor dies." In the case of Queen Elizabeth II, she ascended the throne in 1952 after her father failed to recover from lung surgery and died in his sleep. She was on tour in Kenya at the time, with her late husband Prince Philip.

"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.

"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.

India Hicks, Prince Charles' second cousin whose mother Lady Pamela served as one of the Queen's ladies-in-waiting, told Insider's Mikhaila Friel it may likely be a complicated day for Queen Elizabeth II emotionally.

"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.

"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.