When is best to trek in the Simien Mountains
Trekking in the Simien Mountains is possible at any time of year, however some times are better than others. If you can choose, always opt for September to November. After the summer rains, the landscape is vivid green, the paths are less dusty and the far reaching views undisturbed by haze. December through April is perfectly fine, with loads of sunshine, you are just going to struggle to get picture perfect shots with the hazy views. The rains fall in June, July and August, which tends to stop most visitors, although interestingly enough this time of the year is very popular with the Spanish.
Thanks to the altitude, the temperatures tend to be comfortably warm and sunny in the day with plenty of breeze to keep things cool and cold at night. It is always wise to bring a good three or four season sleeping bag. The locals will always be able to provide something, but you can guarantee it won’t be as warm or clean as your own.
It does tend to get very busy in ethiopia (with hotels booking up and pricing higher rates) during the wonderful religious festivals so if you aren't interested in these you might want to time your trip to Ethiopia to avoid Jan 19-20 (Timkat), Sep 27-28 (Meskel) and Christmas.