In the early years when the Blessed Mother began to appear to the six visionaries in Medjugorje she told them that August 5th is her birthday. The Church celebrates it on September 8th which is nine months after the Solemnity of the Immaculate Conception which is celebrated on December 8th. The Immaculate Conception is about Mary being conceived without sin and not about Jesus.
"The visionary Mirjana also spoke about Our Lady’s ‘private’ birthday in an interview with the author Jan Connell, who wrote in her book, Visions of the Children:
In the first few days of the Apparitions, the Blessed Mother was most gracious about the fear of the six visionaries, and gentle about the personal suffering each was enduring. She told the Children: 'Do you know my real birthday is August the 5th?' The Children said no, they did not know that. The Blessed Mother continued: 'Would you Children like to give me some presents?” Jacov sorrowfully blurted, “Dear Blessed Mother, I don’t have any money!' The Blessed Mother smiled joyfully and said: 'Oh Jacov, my dearest little one, you don’t need money for my birthday presents! Tonight when you go home, no matter what your feelings tell you, no matter how you are stimulated, rather than speak the pain in your heart, run outside and look at the heavens and cry out, ‘Its all for the love of you, Dear Jesus.’”
'''Mirjana said the next evening at the Apparition, the Blessed Mother thanked Jacov for his gifts from the previous evening. By the time the Blessed Mother’s birthday arrived, Mirjana said that Jacov 'was just bursting'. He had billions of presents for the Blessed Mother! High-strung and nervous by nature, young Jacov could hardly contain himself during the long rosary prayers that preceded the Apparition. Finally the moment came. The great flashes of light announced the arrival of the Queen of Heaven and Earth. As Jacov saw her he literally reached up and grabbed at the Mother of God as he cried out. 'Happy Birthday, Dear Blessed Mother!' Realising what he had done, Jacov fell on the floor in shame and fear. Mirjana said she saw the Blessed Mother tenderly lean down and pick him up as she took his little nail-bitten hand in hers and kissed it.'" (http://crownofstars.blogspot.com)