Royals How Queen Elizabeth Is Spending Late Husband Prince Philip's 100th Birthday Today The monarch will be spending the day "privately," a palace source tells PEOPLE By Erin Hill Erin Hill Twitter Senior News Editor, PEOPLE People Editorial Guidelines and Simon Perry Published on June 10, 2021 08:00 AM Share Tweet Pin Email 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. Who Will Stand by Queen Elizabeth's Side at Her First Birthday Parade Since Prince Philip's Death? "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 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. Queen Elizabeth's handwritten letter to Prince Philip. NBC 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. Can't get enough of PEOPLE's Royals coverage? Sign up for our free Royals newsletter to get the latest updates on Kate Middleton, Meghan Markle and more! 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.