Short answer: There is none. If getting a cheap flight were that simple, the (non specialist) travel agent profession would probably have already faltered under it's own weight.
Long answer: See below.
Flight pricing is dynamic. The price begins cheap and then rises with every seat sold and as the flight date approaches before tapering off again (and sometime plummeting) just before departure if the seats remaining exceed the "usual" number of last minute purchases (note that technically all flights are sold beyond capacity; if you know this, assume I'm talking about that imaginary capacity; if you don't, well now you do). Thus, the sooner you book the flight the cheaper it'll be, excluding any campaign discounts or sudden policy, tax or fuel surcharge changes.
A pro tip once passed on to me was to check your calendar for Fridays near your intended departure date. You can almost bank money on the two cheapest regular fares each year on any route that passes through a Western nation being on Friday 13th, departing late evening. Turns out superstition can save you some serious cash if you're lucky.
