When is the best time to come to Ethiopia?

Most parts of Ethiopia are accessible by land or air all year round. However, the ideal time to visit Ethiopia is probably between mid- September and April, when the rainy season is over but the countryside is still green.
You might want your trip to coincide with one of Ethiopia's important religious festivals, particularly Meskel (September) or Timkat (January).
Visiting the historical sites in the north and the east of the country is possible during all seasons, even during the light rains in March-April and the rainy season from mid-June to mid -September, but understand there will be rains for one to two hours in the late afternoon during those times. A good time to take a nap after spending the better part of the day under sunny skies.  
The best times to visit the most southern regions of Ethiopia (e.g. the Omo Valley) are from the third week of June through March.  Some of the most fascinating ethnic rituals like the "Jumping of the Bulls" and the "Evangadi Moon Light Dance" of the Hamer or the stick-fighting of the Mursi and Surma take place mainly during the harvest season from August to October.
The best time for travelling to the Danakil Depression (Dallol) in the north-eastern Afar region, home to active volcanoes, hot springs, colourful minerals, salt lakes and caravans, is from mid-October to March, when the temperatures cool down