The Ethiopian tourist board boasts "13 months of sunshine" but that's not entirely true for the country as a whole. Ethiopia does indeed enjoy 13 months every year, but they're not all sunny. There is a wet season from March through September, and things can get a bit drab, muddy and grey during those months. Despite being close to the equator, Ethiopia's temperatures do fluctuate, depending on altitude. The highlands stay relatively cool year round, so those visiting Ethiopia's "historic route" will enjoy mild temperatures. The lower areas like the Denakil depression will leave you feeling parched and boiled if you choose to visit anytime outside the winter months from November to February (don't worry, you'll still be sweating).
Personally I'd plan my trip to Ethiopia around some of the major Christian orthodox festivals. Meskel and Timkat are the two most lively and colorful festivals on the calendar. It's wise to book a tour far in advance as hotels and domestic flights fill up quickly. Find out more about the Best Time to Visit Ethiopia ...