in most markets it is Tues, Wed and Saturday.........but some cities like Orlando and Vegas are excepted from this rule. Lots of folks like to fly to Orlando on Saturdays so it is usally not the cheapest day to fly.
Often, when there are sale fares to many cities, the sale fare is valid most any day of the week, but Friday and Sunday sell out first, then Mon, Thurs and Sat, leaving Tues and Wed to be considered "cheapest days of the week to fly".