Kilimanjaro is chill all around the year 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.
January, February and the first half of March are the warmest months, with wonderful clear morning skies and evening, making these very suitable time to climb.
June, July and August- are the cold months in Tanzania. Although it’s still warm during the day, nights are very cold.
August and September are peak seasons for avoiding busy trails. September is little quieter and also warmer than August.
Climbs around full moons are very popular for noticeable reasons.
You can experience the rainy season in April and May, which generally means one pretty heavy thunderstorm 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 short rainy season from November to December, although the showers are short but quite refreshing too. October is usually a great time for climbing, getting in just before the rains do, but when there are lesser crowds.