You could wander the Internet for answers, or you could check out a new comprehensive resource from freeshipping.org that tells you when the high and low seasons are in 70 popular destinations. Here's a sampling of a few of the thoughtful write-ups that shed light on weather and prices.
Bora Bora, French Polynesia
Costly: June to August
Less Expensive: April and May
Summer means hot weather and humid, rainy conditions. The weather is more comfortable in April and May than during the height of summer. The humidity isn't as prevalent, and the summer rains have dissipated. Best of all, as the crowds have not yet arrived, you'll enjoy uncrowded resorts, low season rates and an excellent chance of upgrades.
Costly: July and August
Less Expensive: April to June and September to October
The Emerald Isle is known for its rain, so you can expect a certain amount of damp at any time of the year. The weather is likely to be at least as good in May, June and September as in the summer months and hotels are less busy. Spring flowers are at their best in April and May, and October offers a frugal fan's choice of free arts festivals.
Costly: Mid-May to October
Less Expensive: January to March
The Pacific Northwest sees its fair share of rain, but the most popular time to visit is during the summer when braniacs take a break and outdoor festivals are everywhere. Winter is the least expensive time, with fewer crowds and more availability. Second cheapest are the spring and fall shoulder seasons, when the weather is cooler and schools are in session.
For more, see the The Best Time to Go Guide.