The Roman Catholic Church celebrates his feast day not on the date of his death, June 3, as is usual, nor even on the day of his papal inauguration (as is sometimes done with Popes who are Saints, such as with John Paul II) but on 11 October, the day of the first session of the Second Vatican Council. This is understandable, since he was the one who had had the idea for it and had convened it. On Thursday, 11 September 2014, Pope Francis added his optional memorial to the worldwide General Roman Calendar of saints' feast days, in response to global requests. He is commemorated on the date of his death, 3 June, by the Evangelical Lutheran Church in America and on the following day, 4 June, by the Anglican Church of Canada and the Episcopal Church (United States).

Answer this question, if possible (if impossible, reply "unanswerable"): When did Pope Francis added his optional memorial to the worldwide General Roman Calendar of saints' feast days?
11 September 2014