Queen Elizabeth and Prince Philip.

queen elizabeth II

Queen Elizabeth’s beloved late husband,Prince Philip, would have celebrated his milestone 100th birthday on Thursday. The Duke of Edinburghdied 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 forQueen Elizabethafter 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.”

“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.

Prince Philip and Queen Elizabeth.Countess of Wessex

Prince Philip, 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 theQueen placed a handwritten notefor her husband on his coffin — signed with his sentimental nickname for her: Lilibet.

Queen Elizabeth’s handwritten letter to Prince Philip.NBC

A note from the queen placed on Prince Philip’s coffin at the funeral service of Britain’s Prince Philip, Duke of Edinburgh

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On Wednesday, it was revealed that the Queen accepted aspecial 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 toour daily podcast PEOPLE Every Dayfor more on howQueen Elizabethis paying tribute toPrince Philipon what would’ve been his 100th birthday.

source: people.com