HolidayDevelopment

I've been stuck on this, I have all of the song pieces but can't figure out what to do. I've searched every part of Falldown Mall (where it is for me, heard other people have different spots) multiple times but nothing's happening.

Don't know if this question has been asked yet (I checked, but maybe I'm just blind), but what deck(s) do you plan on using for the event? Are you planning on playing for real or just kind of grinding out the event for any rewards (aside from maybe titles)? Any plans on increasing ranking in advance to start on a higher DLv?