To be honest, I am not convinced there is a cheap day to travel (assuming that is what you mean). Of course, as a flight fills up, the price will also rise, but booking far enough in advance, you should have a lot of flexibility when it comes to booking the cheapest fare.
Airlines such as easyJet, Ryanair and British Airways have useful search tools, which show the prices of flights a few days either side of the day you want to travel on. Also try www.skyscanner.net to compare prices every day for a specific month.