April

Usually considered an allusion to Geoffrey Chaucer's The Canterbury Tales where the Prologue starts

1. When April with his showers sweet with fruit
2. The drought of March has pierced unto the root
3. And bathed each vein with liquor that has power
4. To generate therein and sire the flower;

While The Waste Land does indeed start off, like The Canterbury Tales, with an invocation of April and the allusion exists, please remember that Eliot originally started the poem differently.

Read the Prologue to The Canterbury Tales.