It wouldn't surprise me to learn that Friday and Sunday are exceptions - this has been true for years. WN generally does not offer its lowest fares on peak days (except sometimes for early a.m. or late p.m. flights) whereas other carriers often do. So it can be cheaper to fly legacies on Friday/Sunday if you can get the sale fare at your desired flight times. For example WN is currently promoting a $49 fare sale but I haven't even bothered to run some sample itins because I'm almost certain it won't be offered at the dates and times I need.

As I have previously noted at smartertravel.com, such "limited" fare sales are counterproductive because the prospective consumer is inevitably disappointed to learn that the fare is only good for the next month or for travel on Tuesdays and Wednesdays. I certainly don't blame the airlines for trying to maximize revenues in this manner but the result is going to be that fewer and fewer people will jump when the next "sale" is announced, and ultimately those seats will go out empty or will require even further discounting.