Board Thread:Spoilers!/@comment-2153169-20170122203703/@comment-2153169-20170123104217

Eskaver wrote: To be fair to the writers and ABC, noone has said anything or confirmed anything, so it could just be some guesswork by TVLine. Plus, I doubt they'd do a muscial episode, when they'd probably shove that right on the DVD/Blu-Ray edition of ONCE: Season 6. It is confirmed. TVLine would not report it otherwise.

Plus, some people going on the set know it since a moment now. The word was already on the set.

http://katmtan.tumblr.com/post/156238643407/is-the-musical-episode-going-to-be-a-thing-or-is

http://katmtan.tumblr.com/post/156255778372/wait-you-know-its-confirmed-for-sure-or-are-you

So yeah, there is no speculation about that point.