Since last year, the 96-year-old monarch has been experiencing mobility difficulties.
The decision was made for the comfort of Her Majesty, who has suffered from mobility issues since last year.
On Saturday, the Prince of Wales will join spectators at the gathering, which draws spectators from around the world to watch competitors in the caber toss, hammer throw, and tug of war.
Typically, new prime ministers are appointed at either Buckingham Palace or Windsor Castle.
This year, Her Majesty celebrated her Platinum Jubilee, but she skipped several events commemorating her historic 70 years on the throne.
After experiencing discomfort during the first day of Jubilee events, including a birthday parade and flypast, the guest of honor “reluctantly” canceled plans to attend a thanksgiving service at St Paul’s Cathedral.
Concerns about the Queen’s health are growing, as she is frequently pictured sitting or using a walking stick.
Charles, 73, read Her Majesty’s speech while he and the Duke of Cambridge jointly opened the new session of Parliament as Counsellors of State on the Queen’s behalf.
The monarch was also forced to miss her first Royal Ascot since her June coronation.
This year, she declined to attend the annual Easter Sunday service at St. George’s Chapel in Windsor.