Cape Verde is warm and dry all year — Mindelo rarely sees rain. The best months are November through May, when the trade winds are reliable, the air is cooler, and the music scene is at its peak. The summer months (July to October) are hotter and slightly more humid; the brief rainy season, when it comes, is concentrated in August and September.
The single event most worth timing a trip around is the Mindelo Carnival, in February or early March (the week before Lent). It is widely considered the best carnival in Africa, with parades, drumming, and dance contests running for several days. Hotels book out months ahead; book early or stay on Santo Antao and ferry across.