The following announcement was posted on Celtic Frost's official website:

"Celtic Frost are proud to announce their return to North America in spring 2007.
The group has been invited to play the special guest slot on the forthcoming Type O Negative tour in April and May 2007. Due to the preceding co-headliner tour with Kreator in Europe which will conclude on April 5, 2007, in Hamburg, Celtic Frost will not be able to play all dates with Type O Negative. Immediately after the final show of the European tour, however, the group will fly to the United States to join Type O Negative for the first time on April 9, 2007, in Chicago and remain with the tour until its conclusion on May 8, 2007, in Baltimore, MD.
Celtic Frost will be able to play a limited-duration special guest set of 45 minutes duration. In spite of the limited playing time, the band believes that this presents an important opportunity to play for the US audiences again and expand upon the foundation built in the course of last fall's successful 46-date tour of North America."

Source:	celticfrost.com
Band profile:	Celtic Frost
Event:	Type O Negative: US Tour