For me it's Fridays. Based on my riding schedule, it makes sense.. I do a long Sunday, slower recovery Monday. Commute on T/Thurs. Something Wednesday. And on Friday I typically fly - well fly for an overweight 45 year old getting back into serious riding after a few years of mixing in a little riding with running / eating / drinking beer. That said cooler weather and no wind has a major effect. I did a 70 mile ride Sunday that was faster than my average 30 miler from a month ago when it was really, really hot here (like 79-80 at 6:00AM).