Well, every day is a saint's feast day, if you're a Catholic, but it is also the date of the Battle of Agincourt. Both things can be true, you know...
St Crispin's Day features heavily in the speech that Shakespeare has Henry V give to his troops, before the battle ("We few, we happy few...."). The English trounced the French in the battle, so it's ERG-gold. They're probably watching the movie on freeze-frame, while drinking mead, as we speak.