Pope Paul VI
Ecclesiam suam was given at St. Peter's, Rome, on the Feast of the Transfiguration, 6 August 1964, the second year of his Pontificate. It is considered an important document, identifying the Catholic Church with the Body of Christ. A later Council document Lumen Gentium stated that the Church subsists in the Body of Christ, raising questions as to the difference between "is" and "subsists in". Paul VI appealed to "all people of good will" and discussed necessary dialogues within the Church and between the Churches and with atheism.

Q: What was being celebrated in the Catholic calendar when the Eccelsiam suam was given in 1964?
Feast of the Transfiguration