Lagos is hot all year — temperatures rarely below 22°C, rarely above 33°C — and the choice is really between dry and wet.
Dry season (December to March) is the most comfortable time to visit, though the harmattan haze from December into January reduces visibility and coats everything in a fine red Saharan dust. The cool, dry weeks of January and February are also when most of the big music and arts events happen — Felabration in October, the Lagos Theatre Festival in February, Eko Atlantic Festival, and the annual Lagos Jazz Series.
Wet season (April to October) brings heavy bursts of rain, especially in June and September, that flood the lower streets and slow the traffic further. The temperatures are slightly cooler.
August is the lull between the two rainy peaks and is often a surprisingly pleasant time to visit. Avoid arriving on a Friday afternoon if you can: the city seizes up at the start of the weekend.