Shoulder season is the travel period between the peak (or high) season and off (or low) season. For instance, if summer is peak season and winter is off season, then spring and autumn are generally the shoulder seasons.
On the other hand, depending on the kind of trip you want to take, those benefits may not apply. For example, if you’re planning a ski vacation, you need snow—which means a spring trip probably won’t do. And if you’re looking to sunbathe on a sandy beach, autumn temperatures aren’t likely to allow for that.
If you’re just looking for a happy medium of some good prices (though maybe not the lowest), decent (though probably not perfect) weather, and slightly smaller (though not non-existent) crowds, shoulder season can be the ideal time to travel.
Every part of the world has its own shoulder seasons, generally based on weather. In many regions, though, where—as mentioned above—summer is peak season and winter is off season, spring and autumn are both considered shoulder seasons.
That means shoulder season is roughly March–April (early May in some places) and September–October (early November in some places). Shoulder seasons are almost the same months in the northern and southern hemispheres, though the seasons themselves are swapped—autumn in one is spring in the other.
While shoulder season weather conditions in your destination will help determine what kind of trip you’re taking (and can impact on-the-ground costs), the cost of airfare is typically connected to the season in your departure city—not the destination.
Specific shoulder seasons vary by region and country. Knowing when it’s spring or autumn in the hemisphere you’re planning to visit is a good start, but you’ll be much better off with details on the exact places you’re going.