How Queen Elizabeth Is Spending Late Husband Prince Philip's 100th Birthday Today

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Queen Elizabeth and Prince Philip

Queen Elizabeth's beloved late husband, Prince Philip, would have celebrated his milestone 100th birthday on Thursday. The Duke of Edinburgh died on April 9 at the age of 99.

The monarch, 95, who has been staying at Windsor Castle amid the ongoing pandemic, will be spending the poignant day "privately," a palace source tells PEOPLE.

The Queen has been quietly coping with the loss of her husband of 73 years. Philip, who the Queen once called her "strength and stay," quickly adapted to his role as a support for Queen Elizabeth after they wed in 1947.

"There were some things they had to deal with together that were difficult, but it was a long marriage with a great deal of happiness," Charles Anson, former press secretary to the Queen, told PEOPLE. "He took very seriously his role as a support for the Queen as monarch."

Anson says the Queen is now surrounded by the close members of her private household in the wake of her husband's death.

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"She has an enormous amount of family support for her and will also take comfort from the enormity of the support from all over the world," he said.

Countess of Wessex Prince Philip and Queen Elizabeth

The Queen's daughter-in-law, Sophie, Countess of Wessex, said the Queen "has been amazing," following the death of Philip.

The royal family gathered together on April 17 for the Duke of Edinburgh's funeral. It was there that the Queen placed a handwritten note for her husband on his coffin - signed with his sentimental nickname for her: Lilibet.

NBC Queen Elizabeth's handwritten letter to Prince Philip

Meghan Markle and Prince Harry named their daughter, who was born on June 4, Lilibet, in honor of the Queen. Philip is thought to be the last person who called by her childhood moniker.

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On Wednesday, it was revealed that the Queen accepted a special rose in honor of her husband's centenary. It has been planted in an area of the gardens at Windsor Castle that Philip helped transform.

Listen below to our daily podcast PEOPLE Every Day for more on how Queen Elizabeth is paying tribute to Prince Philip on what would've been his 100th birthday.