The best time to visit the DR is November-May, when the weather is calm and reasonably dry, with occasional showers, low humidity and less mosquitoes.
The very best time is February-April, warmer, less crowded and less expensive, apart from February which is Carnival month, with costume parades and varied bizarre celebrations across the island. La Vega is famously boisterous. Events generally climax on Dominican Independence Day, February 27th.
Mid-January to mid-March is also a whale of a time in Samana bay as North Atlantic humpbacks congregate there to mate and entertain thousands of visitors, from shore or boat tours.
The worst time is late July-October when humidity and mosquito-count is high, severe storms are possible, sea will be rough, water visibility poor and beaches strewn with debris.
The DR has a slightly different climate from most of the Caribbean since its north side faces the Atlantic Ocean, but it’s still humid and tropical with average highs of 28C (82F) – 31C (87F) and lows of 21C (71F) – 23C (75F) on the coasts; highs of up to 40C (104F) can occur in sheltered valleys.
Up in the mountains the temperatures are significantly lower and can plummet to 0C (32F) in winter at the highest point, Pico Duarte (3098m).
January and February are the coolest months of the year, while August is the hottest month.
The wet season along the north coast (e. g. Puerta Plata, Samana, Cabarete) may stretch into December or even January, though it’s likely to be just brief daily showers, but the rest of the country gets a traditional Caribbean wet season from May to October/November, with May being the wettest month.
Tropical storms are most likely between August and October. The last time a serious hurricane struck the country was in 1979.