I have always found Tuesday,Wednesday Thursday and sometimes Sunday to be the best days for lowest prices on airfare. Also the fares change around 2:00 AM on Wednesday so since I am a night hawk, I have locked in great fares at that time. They have always got higher after that. At least this has been my experience. However, I have also got good fares only booking two weeks in advance when they want to fill the plane. Sometimes there is just no rhyme or reason for airline pricing!