Nigeria has two seasons: wet and dry. Which one suits you depends on where you're going and what you want to do.
Dry season (November to March) is the most popular time to visit. Temperatures are warm rather than sweltering in most of the country, rainfall is rare, and roads and tracks are passable. The far north can get cold at night in December and January — genuinely cold, down to 10°C — but the days are clear and pleasant. The Harmattan, a dry dusty wind blowing down from the Sahara, can reduce visibility and coat everything in fine red dust from December into February. Carry lip balm.
Wet season (April to October) brings heavy rain, particularly in the south. The Niger Delta and the southeast — some of the wettest places in West Africa — can receive several metres of rain in a year. Roads flood, red-dirt tracks become impassable, and humidity in the south is oppressive. The north has a shorter rainy season (roughly June to September) and remains accessible. Yankari Game Reserve is best visited in the dry season, when animals gather around water sources and the bush is less dense.
Best overall: November through February. January is ideal — dry, not too hot in the south, festivals in full swing across the north during Eid-related celebrations.
December is exceptional if you want to be in Calabar for the carnival. The entire city becomes a festival ground for the month.
Hot season (March to May) sees temperatures peak across the country. The north can hit 40°C or above. This is the least comfortable time to travel.