You never know who might show up at Holiday Pops. During Friday’s matinee performance at Symphony Hall, Ryan McAvoy, a Boston native who suffers from cystic fibrosis, played with the orchestra. (The early Christmas present was arranged by the folks at Make-A-Wish Massachusetts and Rhode Island.) And then in the evening, former Miss Universe Olivia Culpo joined conductor Keith Lockhart on stage to narrate Clement Clarke Moore’s “A Visit From St. Nicholas (’Twas the Night Before Christmas.)” But that’s not all. As Culpo was leaving the stage, she was approached by Pops cellist Ron Lowry, who apparently knew that the 24-year-old model used to play the cello. (Her mom, Susan, plays the viola.) Culpo gamely took Lowry’s instrument and joined the orchestra on a rendition of “The Twelve Days of Christmas.” Cheering Culpo on — and waiting backstage when she was done — was her boyfriend, Patriots receiver Danny Amendola.