Sierra Leone has two seasons: dry (November to April) and wet (May to October).
Dry season (November–April) is the obvious time to visit. Roads are passable, the beaches are at their best, and wildlife viewing in the national parks is easier. November through January is the sweet spot — pleasant temperatures around 28-32°C and minimal rain. From December into February, the harmattan wind blows down from the Sahara, bringing dusty haze and dropping visibility; this affects photography and can make the air uncomfortable. March and April push temperatures up toward 38°C.
Rainy season (May–October) brings heavy, often torrential rain, with July and August the worst months. Secondary roads become impassable, rural destinations can be cut off entirely, and Freetown itself sometimes floods. That said, the country turns intensely green, it's much less crowded, and the beaches look spectacular in the weeks just before the rains end in October.
If your main goal is Tiwai Island or the Gola Forest, the dry season is strongly preferable — access roads are challenging enough without mud. For beach-focused trips on the Freetown Peninsula, you can get away with visiting in late September and October when the worst rains have passed.