The rocker tells PEOPLE all about the “breathtaking” energy of Westminster Abbey

Adam Lambert helped kick off the royal family’s festive season by performing at Kate Middleton’s Together At Christmas carol concert.

Lambert, 41, was among the artists tapped to perform at the Princess of Wales’ third annual Christmas carol service, which was held at Westminster Abbey last Friday. The American Idol alum exclusively tells PEOPLE that it was surreal to sing inside the historic abbey — and shared that he connected with Princess Kate ahead of his performance.

“I had the honor of meeting Catherine, Princess of Wales. She invited the performers she wanted to see at the service, and it was lovely to visit with her before we performed,” Lambert tells PEOPLE.

Lambert sang “The Christmas Song” alongside British singer-songwriter Beverley Knight shortly before Prince William delivered a Bible reading. An attendee said that George, Charlotte and Louis “watched intently” as Lambert and Knight sang the Christmas classic.

According to the artist, the atmosphere inside Westminster Abbey (where the Prince and Princess of Wales wed in 2011 and King Charles was crowned in May) was completely serene when it came time to hit the mic.

“Very still, which added a calmness to my energy. The song felt sort of like a lullaby,” Lambert says.

The “Whataya Want from Me” hitmaker, who first rose to fame on season 8 of American Idol, says that the depth of the setting added to the grandeur.

“I’ve always loved history and old architecture, and this is sort of my sweet spot. I was able to walk around a little bit after my rehearsal and just kind of take a little self-guided tour of some of the areas inside at the Abbey and it’s just breathtaking, and the amount of history in here is nuts,” Lambert says. “You know you have the tombs of monarchs and poets and other iconic historical figures, so it’s pretty wild.”

The “If I Had You” singer says he was honored to be included in the exclusive lineup, which was also highlighted by the Westminster Abbey choir, Jacob Collier, Freya Ridings and James Bay.

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“It’s such an honor to be performing at the Westminster Abbey, a place that I’ve seen on television and in photos for years but have never been to myself, so it is quite breathtaking.”

He was also “excited” to meet the royal family in the intimate setting.

“I’ve performed for the royals during the Queen’s Jubilee but they were quite far away on the other side of the crowd,” he says. “So this will be sort of up close and personal and hopefully bringing Christmas cheer to people that tune in on Christmas Eve.”

In June 2022, the star performed at the BBC’s “Platinum Party at the Palace” concert celebrating Queen Elizabeth’s Platinum Jubilee through the Queen + Adam Lambert collaboration. The musical partnership between the American singer and members of the iconic British rock band opened the show epically held outside Buckingham Palace during the weekend of festivities marking Queen Elizabeth’s record 70-year reign.

Although Together At Christmas was limited to a select guest list on Friday (Westminster Abbey can only seat about 2,200 people!), the public can tune into the Royal Carols: Together at Christmas special airing on ITV1 and ITVX on Christmas Eve, as in years past.

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The latest carol service was linked to The Royal Foundation for Early Childhood’s Shaping Us campaign that Princess Kate launched in January to raise awareness around the early years, and Kensington Palace said that the Dec. 24 broadcast will feature additional content around the topic.

“It’s a really flattering invite to receive from Princess Kate, to come and sing at this special event,” Lamber told PEOPLE, adding that he hoped she enjoyed what they had “in store for her.”