Typically the closer you book to the date that the cruises were released to the market, the less expensive the cruise will be. So for Disney, that is 15-18 months in advance. Because pricing is based on how the sales are going, the more rooms they sell, the higher the price goes.
At the other end of the spectrum - after the Pay In Full date - as people cancelled their ressies (before the PIF)
DCL will sometimes offer "category guaranteed rooms" at a reduced cost - compared to the final prices charged for the cruise. They may more or less expensive than "Opening Day" pricing.
If you live in Florida, they often give last minute discounts, sometimes they give discounts to the Military.
If price is important, then booking a cruise while kids are in school will save money. Spring Break, Summers and even 3 day weekends are more expensive. To me it seems like early September might provide some of the least expensive prices.
TransAtlantic and Panama Canal cruise are also typically less expensive per day since they are two week cruises, during school time, and many people cannot take two weeks off at one time from work.