A local devotion under the title of "Divine Mercy" was approved by the Ordinary of Vilnius in 1936 and from there spread rapidly, especially after World War II in the United States. It appears that Sr. Faustina could not write, save a few lines phonetically. Most of her "diary" was concocted by her sisters after her death. Because of the incongruities of the dairy (different handwriting, different use of terms), the devotion was suppressed, and the book of her diary was placed on the Index Librorum Prohibitorum [Index of Forbidden Books]. This decision was upheld by Pope John XXIII in 1958/59. In early 1978, a Polish cardinal petitioned the Vatican to remove the suppression of the devotion, which was being practiced without sanction in his diocese, and the Vatican replied in the negative, confirming the suppression.
So when was it approved?
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)