Ember Friday in Advent was the original date for this feast. The celebration by the Greeks on July 2 of the placing of the robe of the Blessed Virgin in the Basilica of Blachernae (A.D. 469) became in the Latin church the commemoration of the visit of Mary to the home of Zachary and Elizabeth. It was kept by the Franciscans as early as 1263; the Council of Basil (1431) made it a holy day of obligation.
In honor of this mystery, St. Francis de Sales instituted the order of the nuns of the Visitation. The original design of this kindly saint was that the nuns should imitate the Blessed Virgin in her charitable assistance in the house of her cousin who, though well advanced in years, was miraculously about to become a mother. Providence, however, thought fit to alter the plan while still in the hands of the designer, so that the latter was accustomed jestingly to say that he had made something which he had never intended to make, whereas he could not make that which he would in reality have wished to have made.
(Source: The New Roman Missal, Rev. F. X. Lasance)