Wikipedia: Saint Crispin’s Day falls on 25 October and is the feast day of the Christian saints Crispin and Crispinian (also known as Crispinus and Crispianus, though this spelling has fallen out of favour), twins who were martyred c. 286.
It is a day most famous for the battles that occurred on it, most notably the Battle of Agincourt in 1415. Because of the St. Crispin’s Day Speech in Shakespeare’s play Henry V, calling the soldiers who would fight on the day a “band of brothers”, other battles fought on Crispin’s day have been associated with Shakespeare’s words. Other notable battles include the Battle of Balaclava (Charge of the Light Brigade) during the Crimean War in 1854 and the Battle of Leyte Gulf in the Pacific theatre in 1944.