On 29 May 2006, Celtic Frost embarked on the most extensive tour of the band's career, the "Monotheist Tour", initially headlining festivals (e.g. the Wacken Open Air festival, in front of an audience of 50,000) across Europe the United States and Canada in 2006, and the group's first ever shows in Japan in January 2007. In early 2007 the European leg of the tour took place and a return to the United States as a special guest to Type O Negative. Further festival appearances and concerts followed in mid-2007.