Big parties are said to be in the cards for when Prince William turns 40 years old on June 21 this year, with Queen Elizabeth II already making plans.

The Duke of Cambridge's birthday happens to be after the Queen's Platinum Jubilee celebrations. According to royal biographer Christopher Andersen, she has a "big thing planned" to celebrate her grandson's birthday.

Prince William has something planned as well. The author of "The Brothers and Wives: Inside the Private Lives of William, Kate, Harry, and Meghan" said it is going to be a big summer gathering for the royal family. That is, if COVID-19 restrictions allow.

"The Queen has a big thing planned for William and he is going to throw something as well. There will be big parties again if Covid allows anything like that to happen," Andersen told US Weekly.

It was reported that Kate Middleton had opted for a low-key gathering for her birthday on Jan. 9 in favour of a big party for her husband. Aside from the COVID-19 pandemic, she also thought about the upcoming grand celebrations for Queen Elizabeth II.

The Duchess of Cambridge reportedly celebrated her birthday in Anmer Hall with Prince William and their children, Princes George and Louis, and Princess Charlotte. They invited a few close friends and family and she even baked her own cake.

She also shared a series of lovely portraits with the public to mark her birthday. Royal expert Duncan Larcombe said that the Duchess of Cambridge would rather think of her birthday "as irrelevant to everyone except herself and her family." It was also said that Prince Harry and Meghan Markle greeted her via video call.

Prince William has so far remained mum about what he wants for his birthday. It may be that he and the rest of the royals are preoccupied at the moment with plans for Queen Elizabeth II's Platinum Jubilee celebrations. It is going to be a four-day holiday to celebrate her 70th year on the throne. Regardless of how the Duke of Cambridge celebrates his 40th birthday, fans can expect the customary greetings from the royals to be shared online.

Prince William
Prince William during a visit to the Evelina London Children's Hospital Getty