January, February and the first half of March are the warmest months, with wonderful clear skies mornings and evenings, making these very popular times to climb. Although you may get a few showers during the day.
June, July and August are the “winter” months in Tanzania. Although it’s still warm during the day, nights are cold. Kili is chilly year-round anyway but, if you are combining your trek with other activities, the days will be warm but the nights and early morning safaris are cold.
August and September are peak seasons, so if you want to avoid busy trails you might want to consider another time of year. It is , however, also migratory season for wildlife if you want to add a safari into the already magical mix. September is a little quieter and also warmer than August.
Climbs around full moons are very popular for obvious reasons.
There is a rainy season in April and May, which generally means one pretty heavy rainstorm every day on Kili. However, even that is not guaranteed and it doesn’t last for hours with most of the rain hitting the coast. So although some people want to avoid this, others embrace it, as rain equates with more solitary slopes.
There is another shorter rainy season from November to December, although the showers are short, and quite refreshing too. October is usually a great time for climbing, getting in just before the rains do, but when many of the crowds have gone home.