Guinea-Bissau has two distinct seasons: dry and wet.
The dry season runs from November to May and is the best time to visit. Temperatures are warm (typically 25-35°C), humidity is lower, and roads are passable. This is also the best period for visiting the Bijagos Islands, as boat services run more frequently and the seas are calmer.
The rainy season lasts from June to October, with the heaviest rainfall in August and September. Downpours are intense and frequent, many unpaved roads become impassable, and travel outside Bissau becomes difficult. The Bijagos ferry service becomes irregular. On the positive side, the landscape turns lush and green, and there are fewer visitors (not that there are many to begin with).
February is a particularly good time to visit if you want to experience Carnival in Bissau, the biggest cultural event in the country. The parades, masked dances, and drumming are extraordinary.
December to February is also ideal for birdwatching in the Bijagos, when migratory species swell the already impressive resident populations.