Last minute preparations are being completed and rehearsals undertaken. The Castle has been swept from top to bottom. Security checks are taking place. The men and women of the Armed Forces are applying the final layers of polish to their shoes, blanco to their belts and steaming the sharpest of creases into their uniforms. For days people have been staking out their position on the streets of Windsor to ensure they have the best view of the carriage procession causing, it should be noted, some inconvenience to those who normally sleep in the shop doorways of this pretty Berkshire town.

If, as rumoured, Harry turns up in his Royal Marines uniform it will be a great tribute to his grandfather, Prince Philip, from whom he has just taken over as Captain General, a well deserved tribute to the Corps itself, and a presentational triumph – because the “blues” of the Royal Marines is an exceptionally smart uniform.

Smart as he will look, he will not, of course, outshine his bride.

Meghan Markle will be the star of the show – which is as it should be. For all the apparent broo-ha-ha about her family the run up to the wedding has actually been pretty calm.

Public sympathy is with Miss Markle, not least because of the public affection for Prince Harry. The British have watched him grow-up, make mistakes, serve with distinction in the army, and develop into a passionate and popular member of the senior Royal Family.

St George’s Chapel is the perfect choice for the wedding. An ostentatious wedding at Westminster Abbey would have been a misjudgement. But by Royal standards, a St George’s Chapel wedding is a modest affair. It provides an intimate atmosphere, and convenient dining facilities with the main part of the Castle just up the hill. Most importantly it is a secure location, already behind well guarded walls so it helps to keep the cost down – all smart moves.

Under their feet in the chapel rest the bones and ashes of many of Harry’s forbears. Above them will hang the banners of the Knights of the Garter, that most exclusive and sought after of Royal acknowledgements. Around them will be six hundred of their closest friends and family. The Archbishop of Canterbury will preside well and Bishop Michael Curry will preach very well – and could become an additional star of the day.

The wedding, and it is a wedding because Ms Markle’s first marriage was not religious and therefore not acknowledged by the Church, will be followed by a carriage drive around the town and then a huge party running through the afternoon and evening. It will all provide very good pictures and much newspaper copy, well timed for the Sundays.

But this wedding is more than tabloid fodder, or the basis of an easy joke on the media. Unlike the weddings of his Uncles and Aunt – Andrew, Edward and Anne – or his cousins – Peter, Zara and Eugenie – Harry’s marriage to Meghan actually matters to the future of the monarchy itself. Meghan Markle is not simply marrying the person she loves, central as that is to the proceedings, she is also signing up to be a vital member of the four-person team who will be at the heart of the Monarchy and the worldwide Commonwealth in the decades to come.

William and Harry have forged an incredibly close friendship in addition to being brothers. They have weathered extraordinary family tribulations and public scrutiny and through it all they somehow manage to appear to be engaged and interested in their public duties. They bring a refreshing informality whilst never undermining the essential dignity necessary to successfully carry the role off.

To this duo William added Kate Middleton. She has taken to the role of Princess of Wales in waiting and Queen to be with a deft touch and grace. She forged an independently close friendship with Harry that meant the three of them often undertook public engagements and spent private time together. That Meghan would be comfortable joining this tightly knit grouping must have been a key concern for Harry – and for William and Kate. The evidence seems to be that it all works well.

In the years to come William and Kate, Harry and Meghan will out of necessity form the core of the Monarchy as it moves on – with William and Kate becoming Prince and Princess of Wales and then in time King and Queen. They will need Harry and Meghan to help and support them more, not less as time goes on.

Of course this wedding on the day is about the happy couple splicing the knot in front of the family and friends. For Buckingham Palace, for Britain, and for all of us who care about the survival of the monarchy it is about seeing the person joining the top team who will, for better and for worse, shape the future of a great and essential institution for decades to come.