Travelers dream of the perfect trick to find the cheapest airfare every time, every trip.
There are theories, some of them backed up by real number-crunching — shop on a Tuesday, 57 days before your flight. But using a tidy formula to find the cheapest fare may be as futile as trying to time the stock market.
"There isn't a golden rule anymore," says Patrick Scurry, chief data scientist for Hopper, a travel-information firm that archives ticket prices. "There are these 'average' rules, but they're not that useful necessarily for a specific trip."
Airlines Reporting Corp., which handles tickets sold through travel agents, pinpointed Day 57...