Map from Google - Ivory_Coast

In Ivory Coast, the climate is tropical, with a dry season from December to February, and a rainy season from April to October, due to the African monsoon. Along the coast, the rains are significant also in March and November, and even in December on the westernmost part (see San Pédro, Tabou).
The rainfall is more abundant on the coast, where it goes from 1,500 to 2,500 millimetres per year, while in inland areas it's generally less intense, and ranges from 1,200 to 1,500 mm per year, even though it reaches 2,000 mm in the small western mountainous area.
During winter, the weather is good and the temperatures are high throughout the country, even in December and January, so that the maximum temperatures are around 30 °C on the coast, and 32/34 °C in inland areas. In the latter the humidity is low, since the prevailing wind, the Harmattan, blows from the desert, bringing the dust that can veil the sky and create a characteristic mist.
Here are the mean temperatures of the northern city of Korhogo.
Average temperatures - Korhogo
KorhogoJanFebMarAprMayJunJulAugSepOctNovDec
KorhogoJFMAMJJASOND
Min (°C)182123232221212121212018
Max (°C)343636353331292930323333

Along the coast, the humidity is higher, and some showers may still occur in winter, especially in the western part, and in December, when the sea is a bit warmer and the evaporation is greater. Sometimes, nights can be cool in the interior: lows can drop to around 10 °C. At other times, it can get hot during the day, with peaks of 38/40 °C, even on the coast, at least when the Harmattan, the wind from the desert, arrives here.

Harmattan in Ivory Coast

Starting from February, the temperature begins to increase in inland areas, easily reaching 40 °C. On the coast, the temperature is more stable, even though moisture increases and consequently the air feels more muggy. In April, showers are more frequent along the coast, while in the north the heat is scorching and the temperature can touch 45 °C. Between April and May, the rains begin across the country, but they become really heavy only along the coast, where in May and June, more than 200 mm per month fall, and often also 400 mm. Even the cloudiness is more frequent and compact in the south and along the coast, while in the north, there can be also improvements and periods of good weather.
Here is the average precipitation in Korhogo.
Average precipitation - Korhogo
KorhogoJanFebMarAprMayJunJulAugSepOctNovDecYear
KorhogoJFMAMJJASONDY
Prec. (mm)5105080130140195250205952551195
Days125911121517171131104

While in the centre-north the rains reach their peak between July and September, along the coast in this period there is a decrease in rainfall, especially in August in the central and eastern part, then the rains resume from September to November, without reaching again the peaks of May and June. However, along the coast the sky remains cloudy even in this period.
The period from June to September is also the one with the lowest daytime temperatures, given that they descend around 28/30 °C in inland areas and 26/28 °C on the coast; on the other hand, the humidity is high everywhere, making it feel worse, although the coast enjoys the beneficial blow of the breeze.
In late October and November, with the end of the rainy season, the temperatures begin to rise again in the north-central, while the rains continue to fall on the coast.
In the former capital Abidjan, located in the eastern part of the coast, 1,500 mm of rain per year fall, with a maximum in May and in June, then there's a fairly sharp decline in August and September, and a recovery in October and November.
Here is the average precipitation in Abidjan.
Average precipitation - Abidjan
AbidjanJanFebMarAprMayJunJulAugSepOctNovDecYear
AbidjanJFMAMJJASONDY
Prec. (mm)155512014523537513550110150110451545
Days35911172113121214137137

Along the coast, the temperatures are more stable during the year compared with the interior, as you can see from the averages of Abidjan.
Average temperatures - Abidjan
AbidjanJanFebMarAprMayJunJulAugSepOctNovDec
AbidjanJFMAMJJASOND
Min (°C)232424242423222222232423
Max (°C)313232323129282728293130

Abidjan

In the western part of the coast, the decrease in rainfall during the summer is not so noticeable: in Tabou, where 2,300 mm of rain per year fall, after a peak of 380 mm in May and 500 mm in June, the rainfall decreases to 190 mm in July and 150 mm in August, before rebounding to 265 mm in September. Here is the average precipitation.
Average precipitation - Tabou
TabouJanFebMarAprMayJunJulAugSepOctNovDecYear
TabouJFMAMJJASONDY
Prec. (mm)5055951403804951901502651951751352335
Days45811202116192018159166

The sea is warm enough for swimming all year round, although the water temperature decreases a bit from July to October.
Sea temperature - Abidjan
AbidjanJanFebMarAprMayJunJulAugSepOctNovDec
AbidjanJFMAMJJASOND
Sea (°C)282828292928262525262828

When to go

The best time to visit the Ivory Coast is winter, from December to February. On the coast, especially on the western part, December is also quite rainy, while in the inland areas, in February it begins to get very hot, so all in all the best month for the whole country is January.
In the area of Abidjan, however, in August the rains are not abundant, even though the sky is often cloudy.

What to pack

In winter: in Abidjan and the coast, light clothes for the day, a light sweatshirt for the evening, scarf for the wind-borne dust. In the north, light clothing, a sweatshirt and a light jacket for the evening, scarf for the desert wind.
In summer: loose fitting clothing, light shirts and pants of natural fibres (cotton, linen) or synthetics that breathe, a light sweatshirt and a light raincoat for the rain showers, or an umbrella.