Source: https://www.garabandal.us/prph_great_miracle (verbatim)
The “extremely great” Miracle which Conchita mentions in her Diary is more fully discussed in the notes she gave Francisco Sanchez-Ventura, author of The Apparitions of Garabandal:
“I am the only one to whom the Blessed Virgin spoke of the Miracle. She forbade me to say what it will consist of. I can’t announce the date either until eight days before it is due to occur. What I can reveal is that it will coincide with an event in the Church and with the feast of a saint, martyr of the Eucharist; that it will take place at 8:30 on a Thursday evening; that it will be visible to all those who are in the village and surrounding mountains; that the sick who are present will be cured and the incredulous will believe. It will be the greatest miracle that Jesus will have performed for the world. There won’t be the slightest doubt that it comes from God and that it is for the good of mankind. A sign of the Miracle—un señal del milagro—which it will be possible to film or televise, will remain forever—para siempre—at the pines.”
On September 14, 1965, Conchita said: “The sign that will remain forever at the pines is something we will be able to photograph, televise and see, but not touch. It will be evident that it is not a thing of this world but from God.” At another time, concerning the Great Miracle, she added that, “it would last about 15 minutes.”
On August 10, 1971, while talking to a group of Americans, she offered this revealing information: “It will take place on or between the eighth and sixteenth of March, April or May. It will not happen in February or June.”
Eight days in advance of the Miracle Conchita will give notice to the world about its coming. Prior to the eight-day notice, God will send a Warning to the entire world to purify it for the great Miracle. Therefore, the world will receive two advance notices—the Warning and the eight-day notice.