Angelus Prayer for the Crusades

Most people know that the Angelus is recited at three particular times during the day; 6 am, 12 noon, and 6 pm and during the Easter Season, the Angelus is replaced by the Regina Caeli, a practice first instituted in 1743.

But how many know that the origins of the Angelus is with an 11th century custom of reciting three Hail Mary's during the evening bell which Pope Gregory IX (d 1241) ordered to be rung in order to remind people to pray for the Crusades. Later Pope Callistus III (1455-1458) commended the practice as a prayer for protection against the Turkish invasions of his time. By the sixteenth century the form of the prayer was standardized and it has been highly popular since the 17th century. 

http://www.preces-latinae.org/thesaurus/Cotidiana/Angelus.html