Zambia has three distinct seasons:
Dry season — May to August. The coolest time of the year, with temperatures 24-28°C (75-82 F) during the day, but which can drop to as low as 7°C (45 F) at night. Probably the best time of year to visit Zambia: come early in the dry season for bird watching or to see the Victoria Falls at their most spectacular, or later when the bush is sparse for good game-spotting on safari.
Hot season — September to November. Temperatures can reach 35-40°C (95 – 104 F) but it's a good time for safaris as wildlife congregate around the few remaining watering holes.
Wet season — December to April. Temperatures cool down to 32°C (90 F) or so and some unsurfaced roads may become impassable. Short thunder showers or more protracted downpours are prevalent during these months.
Temperatures do fluctuate based on the altitude, if you are in a valley (such as the Zambezi) it will be warmer and if you are higher up (Kasama) it will be cooler.