Actually that only really applies to parts of the world where Christmas is the major annual holiday. What happens is that a huge number of people fly around the Christmas holiday and then don’t fly so much the next month.
Oh shure, there are ski trips and work trips and some people still holiday in January, but overall traffic levels do drop considerably.
Airlines know this and plan for it, but they have a thing called cash flow to worry about. Even though they may be hugely profitable in December they still need money coming in in January to pay staff, aircraft costs, jet fuel, landing fees, parking fees and a million other expenses that dont dissapear just because fewer people are flying.
One way of improving cash flow is a January offer. You may well see a great offer for a flight provided you pay in January. This is the good old accountants telling marketing to lower their prices, make some offers and get some much needed cash into the tills.
It is harder to tempt people with a discount on a big expense, so generally the better bargains are for short-haul flights. Typically the discounting is around 15% although this varies greatly from airline to airline, depending on how good their accountancy planning has been.
So now you know. Although it might be freezing cold in New York and boiling hot in Sydney, in January there are cheap flights to be found and you can save a lot of money.