This was ethnic, not religious.
The targets were the two main Kurdish political parties, and the Kurdish ambition to become, at least, autonomous inside of Iraq.
The Kurds themselves believe this. The religious celebration was only a way to get into two receptions that the political parties were holding, which was a goldmine of Kurdish political and military leaders.
NPR interviewed Anthony Cordesman of the Center for Strategic and Intelligence Studies on this incident and on the structure of terrorist cells inside of Iraq. Emily Harris reported the story on All Things Considered this afternoon. These are both audio files.
Regards--
Mary Anne