This semantic device [The Day] was not entirely original. Several generations of Southerners had referred to before and after "The War" without being required to explain what war" (123).
The narrator equates the new label for 'The Day' to how southerners used to refer to the Civil War. The main context clue in this passage is "several generations of Southerners" which of course reveals that the referenced war must have taken place at least several generations back; both World War I and World War II had happened too recently, both in the last thirty years, to account for a span of several generations. The Civil War, on the other hand, was a war that rankled deep in the minds of Southerners as being singularly important to the shaping of their history. The Civil War left a deep scar in the South, definitely having the largest impact on the common history of Southerners before 'The Day.'