The best ways to ascertain the on sale dates of popular consumer magazines is either via their media kits or a magazine subscription agency. In regard to the former, all magazines should have an easily downloadable media kit, usually in PDF format, that clearly delineates their on sale dates. This is usually found in the Closing Dates and/or Rates section of the media kit. The other option would be to visit the site of a major magazine subscription agency such as EBSCO http://www.ebsco.com. Most magazine subscription agencies will list their clients, publication frequencies and subscription price but not all will post the on sale dates. You'll just have to check around. Good luck with your search!