I witnessed the festival of Timket in northern Ethiopia in 2004, and consider it (along with the Kumbh Mela) to be one of the most photogenically and culturally interesting religious festivals I've ever photographed. Timket is the most important festival in the Ethiopian calendar, with festivities beginning in the 3rd week of January, the day before Epiphany which, according to the Julien calendar, falls on the 19th of January.
Timket is celebrated all over Ethiopia, but it's particularly spectacular in Lalibela, a Biblical mountain town famous for its 11 churches hewn out of solid rock over a thousand years ago. Many Ethiopians believe they were built by angels. A word of caution: Lalibela is one of the world's dustiest places, and is a haven for flies...other than that, it's a wonderful place! The early dawn prayers in Lalibela are just ethereal in quality and are attended by enormous crowds of devout Ethiopians.
Due to its recent military involvement in Somalia, Ethiopia's current political situation is questionable, so caution is advised.
In the meantime, here are some recent images by Karoki Lewis, a talented photographer, which appeared today on the BBC website: BBC's Timket Festival Images