Burundi sits close to the equator at altitude, which gives it a surprisingly pleasant climate -- warm but rarely scorching, with temperatures hovering around 20-25 degrees Celsius for most of the year. The main variable is rain.
The long dry season from June to August is the best time to visit. Skies are clear, roads are more passable, and the conditions are ideal for visiting national parks and traveling between cities. This is also the coolest period, with comfortable temperatures during the day and cooler nights in the hills.
A shorter dry spell falls in December and January, which is also a good window for travel, though temperatures are a bit warmer.
The two rainy seasons run from February to May (the long rains) and September to November (the short rains). During these periods, expect heavy afternoon downpours. Dirt roads can become difficult or impassable, and travel outside major routes takes longer and requires more patience. The countryside is at its greenest and most beautiful during the rains, but practical travel is harder.
Lake Tanganyika can be visited year-round -- the water is warm regardless of season. If your trip centers on Bujumbura and the lakeshore, timing matters less than if you plan to explore the interior.
Burundi does not have a conventional tourist season. Visitor numbers are low year-round, so you will not encounter crowds at any time. Plan around the weather and your own comfort rather than trying to avoid other travelers.