Nicaragua has a tropical climate with two seasons. This means that in Nicaragua the year is warm, but also that there is the likelihood of considerable rainfall. This rain falls mainly in the rainy season (invierno, winter) which runs from May to October and then there is around 700 mm rainfall. These deposits consist of short, intense, tropical showers that sometimes flood whole streets. The dry season (verano, summer) is from November to April. In this period falls only rain and it can be stifling hot. On the Caribbean coast (east coast) the tropical rainforests are constantly damp. It can rain a lot there even during the dry season. This is in contrast to the Pacific coast (west coast) that has a clear separation between the dry and wet seasons.
The best time to visit Nicaragua is in December or January, because it is a lot cooler. With the exception of mid-April to May, when the country is really dry and dusty, there is not really a bad time to visit Nicaragua.