There are peaks and valleys throughout the year depending on weather and competition among hotels, airlines and cruise lines.
The best time of the year is September and October, which also happens to be the high point of the Caribbean hurricane season. November has low demand / lower prices as well because kids are back in school (as they are in October and part of September). Cruises are cheap then as well.
It also depends somewhat on the destinations. Because Aruba has low rainfall most of the year, it doesn't see the same peaks and valleys in demand as others, although it still bottoms out in September and October.
Holidays usually see big spikes in prices.