Most of the Dominican Republic [1] enjoys balmy weather with average temperatures of 28–31°C (82–88°F) throughout the year. The exception to this climate is in the central mountains, where temperatures can sink to near freezing at night.
The heaviest tourist season is December–February, hiking up prices during these months. In La Península de Samaná [2] the tourist season starts in December and tapers off in March due to prime humpback whale mating season [3]. If you visit in February, try to attend Carnaval celebrations [4].
Dominicans usually take their vacations during Semana Santa (Holy Week), which is celebrated the week before Easter Sunday, making it difficult to book hotel rooms. It is like the Dominican spring break, and many beaches are closed due to the high incidence of drunken accidents. If you want to experience Santo Domingo [5] without all the hectic ferocity that it normally has, Semana Santa is perfect timing.
While it may be tempting to take advantage of cheap airfare in the summer months, the trade-offs are mosquitoes, hurricanes, and extreme humidity. But the pluses are shorter lines and cheaper resort rates. The rainy season runs from May to October and sometimes through December in the northern half of the country. November is a good time because the big wave of tourists hasn’t arrived yet.
Links:
[1] http://www.moon.com/destinations/dominican-republic
[2] http://www.moon.com/destinations/dominican-republic/la-peninsula-de-samana
[3] http://www.moon.com/destinations/dominican-republic/la-peninsula-de-samana/santa-barbara-de-samana/sights-and-recreation/whale-watching
[4] http://www.moon.com/destinations/dominican-republic/discover-the-dominican-republic/explore-the-dominican-republic/carnaval-and-its-masks
[5] http://www.moon.com/destinations/dominican-republic/santo-domingo