Help is at hand. An economist, Makoto Watanabe, has calculated that the optimum time to buy an airline ticket is eight weeks in advance of flying.
His yet-to-be-published findings also suggests that airline tickets are cheaper when purchased in the afternoons, rather than the mornings, prompting him to speculate that airlines are assuming business travellers will book their tickets at work in the morning on the company account, whereas leisure travellers are more likely to book from home in the afternoon
Hmm, I did a couple of searches yesterday but for me, the theory didn’t hold true. But Sunday wasn’t a normal day so perhaps that is the exception.