Why does demand change through the year?

Everybody flies more in the northern summer and again around the turn of the year. Some markets are far more seasonal than others — a beach in August is a different business from a capital-to-capital route in February, and business traffic barely notices the summer.

A pair's seasonality is fixed for the life of the game, so it can be planned around: order in spring for August, and a route that is marginal in November can be the best thing you fly in July.