Peak season is the time when more people are traveling, so prices tend to be higher and crowds are bigger.
Because climates can vary considerably from country to country (and, often, region to region), peak season isn’t the same everywhere.
On a very general level, you can roughly say that peak season is summer—which is June–August in the northern hemisphere and November–February in the southern hemisphere. The Christmas and New Year’s holidays are also peak seasons in many parts of the world.
In addition to those generalities, specific countries or regions or even cities may have their own peak seasons because of popular local festivals, holidays, special events, or microclimates. Knowing when it’s summer in one or the other hemispheres is a good start, but you’ll be much better off with details on the exact places you’re visiting.
Airfare changes with the seasons, as you might expect, with higher demand making for higher prices. What you might not expect, however, is that price changes are based not just on the season in your destination but also sometimes the season in your departure region.
The price differences become particularly pronounced when you’re traveling between hemispheres. For instance, US travelers going to Australia are going to pay much less if they travel in January than in July. Even though June is Australia’s winter and therefore low season, it’s the high season in most of the United States and more Americans are traveling so fares are generally higher.