The Gambia has two distinct seasons. The dry season runs from November to May, with warm days around 30-34°C, cool evenings, and almost no rain. This is when nearly all tourists visit, and for good reason -- the weather is reliable and the birdwatching is superb as European migratory species arrive from late October.
The peak months are November to February, when temperatures are most comfortable and humidity is low. By March and April it gets hotter, pushing above 35°C, though the coast stays breezy.
The rainy season runs from June to October. Rainfall is heaviest in August and September, when short but intense downpours can flood roads and make upcountry travel difficult. Many tourist hotels and restaurants close during the rains, and some river excursions stop running. The landscape, however, turns spectacularly green, and if you don't mind the humidity and occasional disruption, you will have the country almost to yourself.
Birdwatchers should aim for November to March for the best mix of resident and migratory species. The period just after the rains, in November, is particularly good as the bush is still green and the birds are abundant.