I'm not sure if a specific location has ever be canonized. It's implied to have happened somewhere in the Pacific Time Zone, given that the time for it is always given specifically said to be "Aug 17, 2017, at 10:32 AM PST." Also the four volcanoes that the Great Ghost Dance caused to erupt -- Mount Hood, Mount Rainier, Mount Adams, and Mount St. Helens -- are all located in the Pacific Northwest. In fact, on a clear day, I can see all four of them from my front porch here in Portland, OR.
As far as I am concerned, the most likely location for the ritual was Wizard Island, in Crater Lake, in Oregon. This would fit the general themes of Shadowrun's nature versus urban and tie-in to Shadowrun's Pacific Northwest focus. It's relatively close to those four mountains, it's an impressive natural wonder, it's got all kinds of mythology (real and invented) about it, and it would end up right in the heart of one of the more magically inclined members of the NAN: Tir Tairngire.
It would also explain why Lofwyr and the Council of Princes have locked the area down tight (even tighter than the rest of Tir Tairngire which is saying something!). The rumors of massive, apartment sized blocks, covered in geometric symbols, floating in the air over the island -- and the cryptic Shadowland comments from Dunkelzahn and Harlequin about the same.
We (the players) know the Great Ghost Dance punched a hole in reality that nearly let the Horrors through. Dunkelzahn's death was meant to stop them and if you play through certain modules, then your characters can help Harlequin with this too. The Horrors are downplayed in modern Shadowrun metaplots ever since the end of FASA and the brand-split as Earthdawn is not a CGL product. However, the Horrors were always described as quasi-Lovecraftian alien intelligences that Man Was Not Meant To Know. So non-euclidean geometry stones covered in eldritch runes just malevolently floating in the sky sure seems like a Horror-related thing to me.