From a post on Flyertalk.com:
Blackout dates surrounding Xmas & NY's.
Sketchy availability now through summer, but wiiiide open as soon as the kids go back to school (last half of Aug - March 09).
It appears that Mexicana matched US' $208 base fare (before taxes/fees) sale. However, the US fare is only valid for travel through June 20th. If you were looking for a good fare to Cancun for Fall 2008 or Winter 2009 from the bay area, you might want to consider this.
