When to go to Ethiopia ?

The main rainy season is June to end of September. Dont go then unless it is the ONLY period you can take a holiday. it rains pretty well every day - some is drizzle but much is heavy tropical rain. Things get very muddy.  IF you can only go in this period of the Big Rains try to spend some time in lower altitude areas like the Rift Valley Lakes zone, where it will be warmer and a bit less wet....

Best time to visit is mid October to mid March - the dry season. The "Little Rains" come in March / April - but are not heavy or consistent enough to spoil a visit.

May is the hottest month - can get humid in Addis and may rain heavily some days.

August is the coolest month - Addis has temps of 19 C and 11 inches of rain....in August.

All the above relates to Addis and the Central Highlands.  The mainy rainy season is more like 2 months in the north - Tigray ; and 4 monbths in trhe south west. Some areas eg the far south and Omo area......have a different set of weather seasons.