Pentatonix's Mitch Grassi Sounds off on the Group's Christmas Special, the Grammys and Dolly Parton

"It's very reaffirming that the work we've been putting in this year and all these years has not gone unnoticed," says the singer about the nomination

Photo: Tyler Golden/NBC

Christmas is coming early this year for Pentatonix fans.

The musicians are hosting their first-ever primetime Christmas special A Pentatonix Christmas on Dec. 14 on NBC, and it will be equipped with “a lot of singing, lots of performances, some really amazing legendary celebrity guests and a little bit of comedic elements,” vocalist Mitch Grassi tells PEOPLE exclusively.

“Christmas and a cappella just go so well together,” he continues. “I think it’s because a cappella is all about joining together in harmony, just your voices, and Christmas is also about celebrating and being joyful and unity and family and togetherness.”

And joining the a cappella group on their special night will be musical legends Dolly Parton, Reba McEntire and Kelly Clarkson, and Grassi says the group is “so grateful to have people of that magnitude on our show.”

Pentatonix released their second full-length holiday album A Pentatonix Christmas in October after their first Christmas album That’s Christmas To Me in 2014 “did so well,” adds Grassi.

“There’s just nothing like having that quintessential holiday album that you can play with your family year after year, so we wanted to create something like that, but we also wanted to throw in some surprises, so we’re doing some original music like a cover of a Kanye West song, ‘Coldest Winter,'” he says.

Grassi adds of digging up some holiday hits: “It wasn’t easy finding more Christmas songs to put on this album, but I think we found some songs that really inspired us that we could take and make completely our own.”

Once the group wraps up their year, they will be in full-fledged Grammy mode; they were nominated for Best Country Duo/Group Performance for their track “Jolene” with Parton.

“[This nomination] means the world,” says Grassi of the nomination. “It’s very reaffirming that the work we’ve been putting in this year and all these years has not gone unnoticed.”

He adds of working with Parton: “[Dolly’s] just so grounded and so humble. If you didn’t know who she was already, you’d just think she’s this sweet country girl. She’s just so simple and such a happy person. It’s very inspiring. She’s also very professional as well. She’s such a legend in that regard. It never really got to her head. It’s awesome.”

A Pentatonix Christmas airs Dec. 14 at 8 p.m. on NBC.