Benin has a tropical climate, but conditions vary significantly between the coast and the north.
The south has two rainy seasons: a long one from April to July and a shorter one from September to November. The dry season runs from December to March and again briefly in August. The coast is hot and humid year-round, with temperatures typically between 25 and 33 degrees Celsius. The north has a single rainy season from June to October and a long dry season from November to May.
The best time to visit the south is November to February, when the rains have stopped, temperatures are slightly lower, and the harmattan wind from the Sahara keeps the humidity down. January is particularly good if you want to catch the Voodoo Festival in Ouidah on January 10th.
For Pendjari National Park and wildlife in the north, visit between December and May. Animals concentrate around water sources as the bush dries out, making them easier to spot. The park is effectively closed during the rainy season when roads become impassable.
The Somba country and the Atakora mountains are best from November to February as well, when temperatures are pleasant and the landscape is still green from the rains.
Avoid the peak of the rains in June and July if you plan to travel outside the main highways -- unpaved roads can become difficult or impossible. The hottest months are February to April, when the north in particular can be punishing, with temperatures well above 40 degrees.