A festival happening after the seventh day of Sukkot, marked by a service in memory of the dead and a special prayer for rain.
Origin of shemini-atzereth
Hebrew šəmînî ‘ăṭeret eighth (day) of assembly šəmînî eighth
(from šəmōne eight ṯmny
in Semitic roots) ‘ăṭeret assembly
(from ‘āṭar to retain ʕṭr
in Semitic roots)
American Heritage Dictionary of the English Language, 5th Edition