The advice Queen Mary was given before her royal wedding to Denmark's future King, 21 years ago

On this day, 21 years ago, a little-known Australian became one of the most talked-about royals of the modern era.

Mary Donaldson, as she was then called, married Denmark's future King on May 14, 2004 at Copenhagen Cathedral, becoming the Scandinavian country's Crown Princess.

Now, she is Queen Mary of Denmark following her husband's accession to the throne last year.

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Their wedding remains one of royalty's most memorable, with Mary herself describing the occasion as "like a fairy tale".

In 2022, a documentary aired in celebration of Mary's 50th birthday and in it, the royal sat down with journalist Johannes Langkilde to relive some of the biggest moments on her path to royalty.

Over the course of an hour, Mary spoke candidly about her engagement to Frederik, revealing how she learnt to curtsy, meeting Denmark's then-Queen, Margrethe II, and the first time she wore a tiara.

She described the moment her "entire life changed", from being outed publicly as the Crown Prince's girlfriend, several years after their chance meeting at a Sydney pub.

The wedding, of course, was a major focus of the documentary.

Mary became teary when watching moments from the day.

"I'm actually quite moved," she told the program, Mary at 50: You Can Only Be Yourself.

"Everyone was happy. It was truly a celebration of love and future.

"It was big, it was really big. There was a mix of Australian and Danish flags. I was not just accepted for who I am but my background was also accepted, and where I come from. And they took it into their hearts."

Her gown was created by Danish designer Uffe Frank. Mary was given special permission to wear the antique lace veil, becoming the first non-royal bride to wear it.

It was made of an ivory duchess satin and the skirt was lined with 31 metres of tulle. The skirt featured alternating lace panels, which were revealed as she walked down the aisle, followed by a six-metre train.

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Mary wanted her Australian heritage present on her wedding so her bridal bouquet included a flowing trail of eucalyptus, roses and a sprig of myrtle from Fredensborg Palace.

Her father, John, walked her down the aisle to an emotional Crown Prince Frederik who was holding back tears.

They rode a carriage through the streets of Copenhagen with 100,000 thousand people cheering them on.

Upon arrival at Amalienborg, a further 20,000 people had crowed into the palace square to see their first appearance on the balcony as husband and wife and Denmark's new Crown Prince couple.

"When I look at it, I think it was like a fairy tale," Crown Princess Mary said, watching vision of the spectacle.

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"I think every bride, regardless of status, gets the advice to enjoy every single moment. Be present. Try not to lose yourself in it. And I really tried to do that."

She admitted her memories from that day were "a little foggy" and "a little hazy".

"Because it was an overwhelming day and no-one can take it all in."

Speaking about her father Mary said: "When I see my dad I think, 'wow'. He looks as though he is enjoying everything to the fullest.

"He's smiling and he looks so proud."

In the 21 years since marrying into Denmark's royal family, Mary has rarely spoken about her Australian family, who have tried – and succeeded – in staying out of the spotlight.

But putting them through "a wedding of that magnitude", Mary said, was something she felt "guilty" about.

She told the documentary: "My family are people who are firmly down to earth. I felt a little guilty about exposing them to something which for them was so unreal.

"I had gotten used to some of it. But they were happy on our behalf."

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