The actors, who also appeared together in the "Harry Potter" movies, will make appearances alongside Minnie Driver, Rosalie Craig, and Giles Terera in the show, featuring music, comedy sketches and poetry, on 16 December (20).
"Looking a Lot Like Christmas", organised by bosses at the Donmar Warehouse theatre, will be broadcast from St Luke's Church in central London.
While the showing is free, audience members are encouraged to donate money to support the theatre's work, with 15 per cent of money raised going to the Connection at St Martin's homeless charity.
"At the end of a very difficult year for us all, we wanted to bring together a group of incredible artists to celebrate the festive season in true Donmar style," Donmar Warehouse artistic director Michael Longhurst said.
"Even though we can't be together in person, we hope you will all join us online to share the joy of music and theatre which has endured throughout the challenges of 2020."
Tune in from 7.30 pm GMT here: youtube.com.
Another Christmas online event comes from Andrea Bocelli. The Italian tenor has announced "Believe in Christmas" show. He is scheduled to perform live from the Teatro Regio di Parma opera house on 12 December (20) with special guests.
Before the live performance, Bocelli will join Disney holiday singalong on November 30. It features the likes of Katy Perry, Pink, BTS, Michael Buble, Chloe x Halle, Derek Hough and Hayley Erbert, Julianne Hough, Adam Lambert, Leslie Odom Jr., and Kerry Washington.