More specifically, the period called "off-peak" is the Best Time to Buy Business Class Tickets to US if it is to the US one is flying. With other words, the months late into winter (January to March, to be more precise) are generally when declines in demand for air travel directly result in cheaper fares. Also, those cheap tickets are more often sold midweek, particularly Tuesdays and Wednesdays, when promotional activities typically target these days.
If it is on a business class ticket, securing a trip to the USA several months prior already cancels the fares to a certain level. Any ticket priced lowest in business class must be bought between 6-8 months ahead of departure; yet, even if you lose the chance for any reason, there still remains a prospect of getting a substantial discount.