A number of these visions of the Virgin Mary have been reported, although only a few of these have been declared by the Catholic Church as "worthy of belief." The three best known are (1) the appearance at Guadalupe, Mexico in 1531 to the Indian peasant Juan Diego; (2) the appearance at Lourdes, France in 1858 to Bernadette Soubirous; (3) the appearance at Fatima, Portugal in 1917 to three peasant children.
Of the three, Fatima was the best documented because it was witnessed by an estimated crowd of 70,000. At Fatima, the lady appeared to the children on May 13, 1917, and again on the 13th day of each month until October 13th. Word of the appearances had gotten out and a larger crowd had come each month. The lady had promised a miracle on October 13. On that day it was raining hard but nevertheless a crowd of some 70,000 had gathered. What they saw was the clouds open, followed by abnormal movements of the sun. After some 10 minutes, the sun stopped moving but the ground and the people's clothing were dry. For a detailed account, see here:
http://www.theotokos.org.uk/pages/approved/appariti/fatima.html
I know most people here are skeptical of this kind of thing and I have doubts myself. However, I have spent a great deal of time studying the history of the Fatima apparitions and it is not possible to just dismiss them as not happening.
So what say you?