In the Catholic Church, Mary has several feast days. Being the human Mother of Jesus, her maternal relationship is real and permanent. Therefore, just as any son or daughter would honor special occasions in the life of his or her mother, so, too, the Church observes the unique events in the life of the Mother of the Savior. Following is a quick rundown:
Her birth is celebrated on September 8 in the Latin Church. The Church only celebrates three earthly birthdays: Jesus Christ on December 25; St. John the Baptist on June 24; and the Virgin Mary's. The feast days for the other saints usually mark their heavenly birthdays — that is, the day they died. Mary's earthly birthday is important because Jesus would have honored his mother's birthday.
The Church celebrates Mary's conception just as it celebrates Jesus's. December 8 is the Solemnity of the Immaculate Conception, flowing from the dogma of the Immaculate Conception defined by Pope Pius IX in 1854. This dogma teaches that by a special grace from God, the Virgin Mary was preserved from all sin, even original sin, from the moment of her conception so that when she grew up, she could become the Mother of the Messiah. As such, she would need to be free from sin so she could give Jesus an untainted human nature.
Mary's Assumption into heaven is celebrated on August 15. This day marks the taking up of Mary's body and soul by Jesus. (The Ascension is when Jesus himself took his body and soul into heaven 40 days after Easter. The Assumption is when he took his mother's body and soul to the same place.)
The Motherhood of Jesus is celebrated on January 1. It was formerly the Feast of the Circumcision, but the Second Vatican Council wanted to begin the civil new year by honoring Mary, the Mother of God.
Mary's Queenship is celebrated August 22. This feast is relatively new on the calendar, but the title is ancient. It's an extension of the honor due to Christ the King and by extension to Mary, who is the Mother of the King. As Queen Mother, she is honored, venerated, and highly respected above all other humans and angels.
Numerous other Marian feasts honor various apparitions (appearances of Mary) such as Lourdes, Fatima, Guadalupe, Knock Ireland, and so on. Other feasts associated with Mary, such as Our Lady of Mount Carmel (July 16) and Our Lady of Seven Sorrows (September 15), are secondary to the major ones listed here.