Statement from ISIS said attack on APOSTATE Holiday and Turkish military.
Read Carefully, I know its hard for some they only want to read certain things that fit their agenda.

The Islamic State in Iraq and Syria claimed responsibility on Monday for a New Year's Day mass shooting in a packed Istanbul nightclub that killed 39 people, an attack carried out by a lone gunman who remains at large.
It described the Reina nightclub, where many foreigners as well as Turks were killed, as a gathering point for Christians celebrating their "apostate holiday." The attack, it said, was revenge for Turkish military involvement in Syria.