Her absence Tuesday is also only the third time it has happened in her 70-year reign. She missed two previous openings of Parliament during her pregnancies with her two youngest children, Princes Andrew and Edward.
This time, the 96-year-old queen's absence was due to her health, specifically mobility issues, according to Buckingham Palace.
"The Queen continues to experience episodic mobility problems, and in consultation with her doctors has reluctantly decided that she will not attend the State Opening of Parliament tomorrow," the palace said in a statement Monday. "At Her Majesty’s request, and with the agreement of the relevant authorities, The Prince of Wales will read The Queen’s speech on Her Majesty’s behalf, with The Duke of Cambridge also in attendance."
On Tuesday, the queen's crown held her place at the opening of Parliament. It was placed next to Prince Charles, the queen's oldest child and heir to the throne, who sat on the Consort Throne.
Joining Charles were his wife, Camilla, the Duchess of Cornwall, who will become queen consort when Charles becomes king, and Charles's oldest son, Prince William, the second-in-line to the throne.
The last time Queen Elizabeth was seen publicly in person was in March, when she led the royal family at the Service of Thanksgiving at Westminster Abbey for Prince Philip, her husband of over 70 years.
The queen entered Westminster Abbey using a walking stick and holding on to the arm of her son, Prince Andrew, who in February agreed to settle a sexual assault lawsuit.
Her appearance came after she had battled several health conditions over the past year, including COVID-19 and an overnight hospitalization for what Buckingham Palace described at the time as "preliminary investigations."
While Queen Elizabeth has continued to maintain a busy schedule of virtual meetings, phone calls and private engagements, other members of the royal family, including Charles and Camilla and William and his wife, Duchess Kate, have taken on more of her public duties.
"We know that from any family, when a matriarch is getting older, others do what they can to help out, and that's exactly the same with monarchy," said ABC News royalty consultant Alastair Bruce. "I think for the queen, we all understand she's 96. ... She has been monarch for 70 years, and I think it's very understandable that at this stage of her life, she should have a right to choose when she wants to go out and take part on the significant occasions or when she has one of her family to do it."
It remains unclear how many events the queen will attend for her own celebration.
"Palace aides understand that the queen at 96 will do what she wants to do, what she feels able to do, and in the end, she will know exactly what the plans are," said Bruce. "The way they're dealing with it at the moment is each day the queen makes a decision in the morning, will she get involved or not, and let's see what she decides."