We think Kayak.com is the best site for flights research, now that travel.bing.com is gone (actually long gone).
Select “+-3 days” to see how one or two days could drastically change your flight costs. The basic rule is, flying Wednesday to Wednesday is usually the cheapest, with weekend flying being most expensive.
Select “Explorer” to see when the cheapest times are to fly. Mid-Feb thru mid May is the cheapest time to go to Hawaii (no surprise).