Most Caribbean islands have only two seasons: tropical and…a slightly cooler version of tropical. Puerto Plata is no exception. Temperatures across the Dominican Republic are pretty constant year round, with averages in the mid-80s during the day and in the low 70s at night.
Humidity is the main factor that sets Puerto Plata's slightly different seasons apart. The tropical season, which runs from May to November, gets the bulk of the town's precipitation, which makes the air a bit wetter and sweatier overall. Even if the actual temperature in the tropical season is the same 70 degrees as it is in say, December, it feels a whole lot hotter.
In November, the rain and humidity dissipate, and things stay cooler through April. This so-called "tropical cool season" is the sweet spot. Keep in mind, though, that this is also the Caribbean's high tourist season, especially during the December holiday periods. That higher-traffic flow means that most hotels and resorts raise their prices and book up further in advance. Some places, in fact, as much as triple their rates in the two weeks around Christmas.