"Hooper had wept often, but never for Henry's speech on St. Crispin's day"

The speech made by King Henry V of England in Shakespeare's play, Henry V (Act V, Scene 3).

The play deals with events surrounding the Battle of Agincourt, one of the battles of the Hundred Years War, and the speech referred to is delivered by Henry V in order to motivate his troops.  It is the origin of the phrase "band of brothers".

St. Crispin's day is a Christian feast day, celebrated on October 25.