WHEN is the best time to book that flight? It’s one of the most fraught decisions travelers face, as ticket prices often fluctuate right up to departure time.

Recent fare analysis by the Airlines Reporting Corporation seems to challenge the conventional wisdom that the earlier you book, the less expensive your fare will be. In January, the corporation, which processes ticket transactions for airlines and travel agencies, reported that over the past four years passengers paid the lowest price for domestic flights when buying just about six weeks in advance.