The dogma of the Immaculate Conception holds that Mary, the mother of Jesus, was free from Original Sin from the very moment of her conception. The Immaculate Conception of Mary is often confused with the virginal conception of Jesus.
Today's feast was first approved by Pope Sixtus IV in 1476.
In 1830 St Catherine Laboure experienced a vision in which she saw Our Lady standing on a globe with rays of light emanating from her hands. The vision was surrounded by an oval frame on which were the words 'Oh Mary conceived without sin pray for us who have recourse to thee.'
In 1854, Pius IX solemnly decreed that 'the most Blessed Virgin Mary was, from the first moment of her conception, by the singular grace and privilege of Almighty God, and in view of the merits of Christ Jesus the Saviour of the human race, preserved immune from all stain of original sin. That is revealed by God and therefore firmly and constantly to be believed by all the faithful.'
Just four years later, in Lourdes, the 14 year-old St Bernadette, who had very little education and would have know nothing of papal statements, began to experience a series of apparitions of 'a lady', When she asked her name, the Lady replied: "I am the Immaculate Conception."
Saint Bernard wrote of Mary: "If you follow her you do not stray; if you think of her your mind does not err; if you ask of her, you will not be disappointed; if you cling to her, you will not fall. If she be with you, all is well, and you will realise the truth of the words 'the name of the virgin was Mary.'
Watch: Pope Francis speaking about the doctrine of the Immaculate Conception on this day in 2013: