It depends on what is important to you. Are you looking for the best weather or the best price? Do you want to see the whales in Hawaii or the nesting sea turtles in Costa Rica? This is important information when you're planning your hard earned vacation.
High season is inevitably when the greatest number of people want to travel to a destination and low season when the least number of people want to go. It seems obvious you're going to pay more during high season and less in low season, you know, supply and demand. When hotels and airlines know that demand is high they can charge a premium. They know they'll fill their rooms and seats so that is the time to maximize their profits. During low season they offer discounts so rooms and seats don't sit empty.
There is another option here. Shoulder season has become more popular as high season prices have risen. Shoulder season is typically just before and after the crowds surge to a destination. To avoid the least attractive aspects of low season travel and the high season prices, shoulder season is a good alternative.
For years, the fall was a bargain in Europe. The summer crowd and the searing heat were gone and the cooler weather and shorter lines could be expected. That's changed a little over the years and fall isn't the bargain it used to be but for my money it's the best time of the year to go.
There are exceptions, Octoberfest in Germany will cost as much as high season, maybe more. Easter in Italy might be a little chilly but it's prime season in Rome and St. Patrick's Day in Ireland might have limited availability so my tip here is to book early. If there are low rates they'll be booked first.
If you're feeling the sting of Spring Break prices get a jump on it next year and book as soon as the fall semester starts. Most people begin booking spring break after the winter holidays but you'll find the best prices booked in October and November for spring travel.
Sure, pricing and the weather aren't the only things to consider in planning your vacation. Sometimes it just comes down to when you can go so talk to your travel agent to find the best options for you based on whats important to you.