Warning This Section Does Contain Spoilers
In the Friend letter is the quote:
"Whan that April with his showres soote the droughte of March hath perced to the roote, and bathed every veine in swich licour, of which vertu engendered is the flowr."
This comes from the General Prologue of The Canterbury Tales meaning:
"When in April the sweet showers fall that pierce March's drought to the root and all and bathed every vein in liquor that has power to generate therein and sire the flower."