Ethiopia's climate is determined more by altitude than by latitude. The highlands, where most travelers spend their time, are pleasantly mild year-round — warm days and cool nights at elevations above 2,000 meters. The lowlands and the Danakil are brutally hot.
The main rainy season, called kiremt, runs from June through September, with the heaviest rains in July and August. During this period, unpaved roads can become impassable, trekking in the Simien Mountains is difficult, and the Danakil Depression tours do not run. Some travelers avoid these months entirely.
The best time for most itineraries is October through March. October and November are particularly good — the rains have just ended, the landscape is green, wildflowers are blooming, and the air is clear. December through February is peak tourist season with dry weather and comfortable temperatures in the highlands, though nights can be cold at altitude.
The short rains, called belg, fall between March and May, mostly in the south. They are lighter and less disruptive than the main rains, but can still cause delays on rural roads.
For the Danakil Depression, go between November and February when temperatures are merely extreme rather than unbearable. The Omo Valley is best visited from September through March, outside the rains that make the roads difficult.
If you want to time your visit around a festival, Timkat (Ethiopian Epiphany) in January is the most spectacular — a three-day celebration with processions, music, and the blessing of water in every town. Meskal (Finding of the True Cross) in late September features enormous bonfires and marks the end of the rains. Both are extraordinary experiences.