This is a question frequently asked; fortunately there is no bad time to visit Ghana. The weather is very nice all year round and does not experience any extreme weather like hurricanes or tornados, and absolutely no reason to pack a winter coat. There are two different seasons there is the rainy season and the dry season, each season occurs twice a year.
January to March- Dry Season. Max temp 88F, Min temp 73F
April to June- rainy season. Max temp 84-88F, Min temp 73F
July to September-Dry season. Max temp 81F, min temp 72F
October to December- Rainy season. Max temp 88F, Min temp 73F
Even with these seasonal changes Ghana is relatively hot and humid all year round. Being close to the equator (between 4-11 degrees North) the temperature does not fluctuate too much; neither does the length of the day. In the rainy season there is usually one rain storm in the afternoon rather than it raining continuously all day every day.
When to visit really depends on what you want to do on your trip and where you are going. If you are traveling the whole country and going to the north to see wildlife December through April is a good time to go being the dry season. You will not have to worry about the roads being too bad and the grasses are lower in the north to see the wildlife. There is the added bonus that the most of the watering holes have dried up also so it is much easier to find the wildlife at the remaining watering holes. The downsides are that the heat in the north can be tiresome, and during this time of year a phenomenon called Harmattan is going on. This is where the winds are blowing down from the Sahara desert bringing with them fine sand particles in the air. The sky looks dirty and hazy almost like a fog, when it is bad this occurs throughout the whole country even the south, often it is worse in the north though. This is no big deal other than it makes getting pictures with the right lighting difficult. Also it will restrict views if you are doing much hiking or viewing from mountains. So if photography is not one of your things visiting during this time is great. It can be quite cool at night during the dry season also, especially the further north you go, it is recommended that you bring a sheet or something to cover up with to sleep.
If you are staying in the south of the country the rainy season can be a great time to visit. The roads are not as much of an issue because they tend to be better in the south. Also along the coastline it is usually a little drier than the interior. If you anticipate the afternoon rain storm and take a nap for an hour or relax during the rain, most of the day is very usable and you can enjoy the sunshine and blue Skies. The wettest months of the year are May, June and October. During North America’s summer is a great time to visit the South of Ghana. The temperatures are cooler than what you would experience back home but still warm enough to enjoy the water and be outside all day and night. During July and August there is often a lull in the rains which combined with the cooler weather makes for paradise. Because of the clear skies other than during the rains this is a better time of year to do photography. Everything is much greener, even in the north which is more aesthetically pleasing.
Generally though you cannot go wrong visiting Ghana any time of the year in terms of weather. There are festivals in different areas of the country year round, but during August and September in the south of the country there is a higher concentration of festivals. It is generally recommended that you probably don’t want to be in Accra during the Christmas Holidays, they call it the city of Ghosts at this time because there are so many people that aren’t there other times of the year, many people are coming to the city to do shopping, also many Ghanaians that are usually spread out through the diaspora return home for the holidays. Everything is congested and traffic can be a nightmare, also officials and officers are looking for an extra payout or opportunity to get a bribe to have a little more cash in their pockets for the holidays. Spending this time outside of the city on a remote beach somewhere is the best way to do it, often there will be some festive parties and celebrations with lots of dancing and music. Just get your reservations in early this is the High season for tourism.