This day is called St. Crispin's Day, the Feast of St. Crispin and Crispinian. Of course, today is the anniversary of the Battle of Agincourt in 1415AD, made famous by the great speech of King Henry V of England in William Shakespeare's play by the same name.
For more on St. Crispin, click here: Catholic Saints Info: Saint Crispin
The martyrs St. Crispin and Crispinian are, today, buried in the Roman Church of San Lorenzo in Panisperna...
As for the battle of Agincourt, fought on this day, it is well to recall the particular context and result. In 1415, Henry V (1413-1422), asserting his claim to the throne of France, began his invasion of France, this despite Emperor Sigismund’s attempt to make peace. He departed in August, and by the end of September, had the city of Harfleur. Exhausted, his army tried to march north to Calais to return to England, but was attacked by the French Royal army of King Charles VI (1380-1422) on the way – this at the field of Agincourt on 25 October 1415.
Map of the Battle of Agincourt, 25 October 1415AD.
The English were outnumbered four to one. The French knew of the English longbow, dismounted, and were clubbed and arrowed to death in the muddy field. The loss were huge: several notable officials, including the French commander, Constable Charles d’Albret, who commanded the French on the field; 90 counts, 1,500 knights, and 4,000 others. Also several Dukes, like Bourbon, were captured. The English lost practically nothing. [About 450 men?]
Certainly the play, Henry V, of William Shakespeare, memorably presents the English force in the moments before facing the superior French force. This from Act IV, Scene III: