This will be the seventh Royal ceremony that the Queen has led over the last 22 years, marking the political changes in Wales brought about by devolution.
It'll be a day of music, pomp and celebration but poignant too, as the first she will have attended without Prince Philip.
When she opened what was then known as the National Assembly for Wales in May 1999, she described it as "a bridge into the future."
The institution that she opened at that time was more of a step than a bridge, with limited powers and meeting in a repurposed office block in Cardiff Bay.
She returned to the theme four years later when she opened the second session of the Assembly, telling members that: “Bridges span divides; they unite people. As elected Members, these are roles which you fulfil.
"The essence of representative democracy is that elected Members are the means by which the wishes and needs of the citizen are articulated to government at all levels on those matters for which they are responsible. You are bridges between the people and the devolved government of Wales."
The Queen has been present at the start of every session of what is now the Senedd or Welsh Parliament, following six elections.
She also came to open the Senedd building itself, when the members, then still known as AMs, moved into their purpose-built home in Cardiff Bay.
For this, her seventh visit, you can expect her to speak more of unity and bridge-building and likely to emphasise, as she has previously, the permanence of devolution.
That may be something that not everyone in Welsh politics is wholly signed up to, but the monarch has been clear over the years, using her speeches to state how she has "embraced" devolution and supports it as a way of governing the UK.
Now on an autumnal October day in 2021, there will be a 21-gun salute to mark her arrival in Cardiff. There will be fanfares and musical performances as she and the Royal Party arrive in Cardiff Bay.
As happens in the UK Parliament in Westminster, the ceremony will focus on the mace, a ceremonial object long-linked with parliamentary democracy.
The mace belonging to the Senedd is made of gold, silver and brass and was presented to it by the Parliament of New South Wales to mark the opening of the the Welsh Parliament building in 2006.
As in previous years, it will be carried in a procession by a member of the Senedd's security team, this time Shahzad Khan, who says the role will be an honour that he can't describe.
He said: "When I found out that I'm actually going to be the mace-bearer - wow! Once in a lifetime really."
The Queen will be accompanied by the Prince of Wales and the Duchess of Cornwall but it will be her first visit to Wales since the death of Prince Philip.
When he was in Wales, the Duke of Edinburgh was known as the Earl of Merioneth, a title his wife gave him when they married and the title she used during Royal Openings, even when speaking directly to him. Not 'Philip' but 'Merioneth.'
The opening ceremony itself will take place in the Siambr, the debating chamber of the Welsh Parliament.
Once the mace is put back in its usual place, symbolising the official beginning of the Sixth Senedd, the Queen will make a Speech as will the Llywydd (Presiding Officer) Elin Jones and the First Minister, Mark Drakeford.
She'll then sign a commemorative parchment. There will be more music and poetry reading before the Royal Party leave the Senedd, although not before they meet a group of Covid Community Champions, people nominated in recognition of their roles during the coronavirus pandemic.