Vespers

Vespers is the evening prayer service in the Roman Catholic, Eastern (Byzantine) Catholic, and Eastern Orthodox, liturgies of the canonical hours. The word comes from the Greek εσπερινός and the Latin vesper, meaning "evening." The term is also in limited use in some Protestant (especially Lutheran and Seventh-day Adventist) denominations to describe evening services, and in some Anglican circles is used unofficially to refer to Evening Prayer.

http://en.wikipedia.org/wiki/Vespers

Rachmaninov's Vespers is written in Church Slavonic.