Susan Boyle said that her chance to sing in front of the Pope at an open-air Mass was "beyond her wildest dreams".
The 49-year-old singing sensation is expected to perform three songs, including her smash hit I Dreamed A Dream, which catapulted her to fame on TV's Britain's Got Talent.
Susan, from Blackburn in West Lothian, has been rehearsing at Bellahouston Park in Glasgow, where the open-air Mass will be held.
Speaking about what the event meant to her, she told BBC Reporting Scotland: "It's hard to describe it. It's hard to put into words, really. But it will be a very humbling experience, a very indescribable experience. It's beyond my imagination, beyond my wildest dreams."
Speaking about her late mother, Bridget, she said: "It will be very poignant tomorrow. She died about four years ago but she will be here in spirit and, I think she will be very proud."
She said that, on a scale of one to 10, out of the concerts she had played the event was an "11".
"Again, you can't really put it into words," she added.
When asked what exactly she would be singing, she said: "Watch and see. Big secret, big secret."
Speaking about the thousands who will be at the Mass in Bellahouston Park, she said: "I'd like to think I am making that day extra special for them."
In the first state visit by a Pope to Britain, Pope Benedict XVI will meet the Queen and Prime Minister David Cameron during his four-day tour.