Fr. Z seems to know: 19 September 1846: Our Lady of La Salette.
The message has been added to and embellished over the years, although some believe the messages from Akita, Japan say the same thing - more or less. (Approved La Salette message here.)
I read today that the seers at Medjugorje have said that official recognition by the Vatican will not come until after the visionaries are dead.