The northern hemisphere's winter is high tourist season in the Dominican Republic; this is when the Dominican climate is at its optimum, having cooled down just a bit. You'll therefore save a bit of money - and have an easier time booking a hotel room on the spot - if you arrive during the spring or the fall, which is just fine, as the temperature doesn't really vary all that much from season to season. Keep in mind, though, that the Dominican Republic is right in the centre of the Caribbean hurricane belt, and gets hit with a major one every decade or so; August and September are prime hurricane season, though smaller ones can occur in the months preceding and following.