Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Dallas to Cotonou as you normally would.
Due to multiple factors, expect to potentially pay more when booking in December. For low rates, February can be a great choice for travel to Cotonou. Flights to Cotonou from Dallas in February are about $1,216 on average during February, but can be found for as low as $936.