I noticed a more than $200 difference in PHX-ATL round trips between leaving on a Thursday, Friday or Saturday vs. leaving on a Sunday or Monday and coming back after 4 days or so. This is independent of the week one flies. The requirement of Saturday night stayovers for halfway decent fares on domestic had been eliminated quite a few years ago, but it seems Delta is testing the waters to see whether they can revive this requirement and have business travelers pay through the nose. Right now I noticed it just for PHX-ATL; hope it does not spread....