In the video, fans see original stars Robbie Coltrane (Rubeus Hagrid), Matthew Lewis (Neveille Longbottom), and Mark Williams (Arthur Weasley) receiving their handwritten invitations and getting ready to board the Hogwarts Express.
"The invitation you've been waiting for has arrived," the clip teases.
The credits also list original stars Daniel Radcliffe, Rupert Grint, and Emma Watson as reunion attendees, as well as Helena Bonham Carter, Ralph Fiennes, Gary Oldman, Imelda Staunton, Tom Felton, James Phelps, Oliver Phelps, Bonnie Wright, Alfred Enoch, and more.
The highly anticipated event will stream at midnight on New Year's Day, and will mark the first time the original cast has reunited for a televised event since the 2011 release of the franchise's final film, Harry Potter & the Deathly Hallows Pt. 2.
Last month, Watson gushed about the franchise on Instagram ahead of the anniversary special.
"I am proud not just of what we as a group contributed as actors to the franchise but also as the children that became young adults that walked that path. I look at my fellow cast members now and I am just so proud of who everyone has become as people. I am proud we were kind to each other that we supported one another and that we held up something meaningful," she wrote. "Happy 20th Anniversary Potterheads! We hope you enjoy the Harry Potter 20th Anniversary: Return to Hogwarts which will stream New Year’s day, Jan. 1 on HBO Max. ⚡️ Lots to look forward to. 😊"