Although times vary by destination, “shoulder season is the travel period that falls squarely between the peak travel season, also known as the traditional summer months of June through August (for the northern hemisphere ), and the low season, which is generally in the middle of winter (January to early March, in the northern hemisphere),” according to AFAR.
Like planning an off-season flight, you'll need to be wary of weather changes when booking a shoulder season trip, but if you're willing to think off the beaten path, you'll find more affordable flights and lower hotel rates. and less congestion.