So when was it approved?
It was approved by JP2 shortly after he became pope. From what I had read, he had been trying to get his predecessors to approve it for years prior to acceding to the papacy.
Considering that private revelations are non-binding. We can agree to disagree about its authenticity.
Byzantine Christianity is 100 percent opposed to "private revelations" of any kind. Actually, the Roman Church's acceptance of it is an ecumenical obstacle with the Orthodox.
April 15, 1978
The Sacred Congregation issues a new "Notification" which states: "This Sacred Congregation, having now in possession the many original documents, unknown in 1959; having taken into consideration the profoundly changed circumstances, and having taken into account the opinion of many Polish Ordinaries, declares no longer binding the prohibitions contained in the quoted "Notification" (of 1959)." (copied from Now is the Time for Mercy [page 99] by Rev. George W. Kosicki, CSB)
April 15, 1978 to October 16, 1978 --- 6 months