Month of the Immaculate Conception

The Month of December is dedicated to the Immaculate Conception, and includes a holy day of obligation for the Solemnity of the Immaculate Conception. 

From Catholic Culture:

The month of December is traditionally dedicated to the Immaculate Conception. The Blessed Virgin Mary, in the first moment of her conception, by a singular privilege of Almighty God, and in view of the merits of Jesus Christ, our Savior and hers, was preserved from all stain of original sin. This age-old belief of the Church was defined by Pope Pius IX in 1854 as an article of revealed truth.

Mary was in need of redemption and she was indeed redeemed by the Precious Blood of Jesus Christ. The manner of Mary's redemption, however, was unique. Instead of being freed from original sin after having contracted it, she was preserved from contracting it. This was a most fitting favor for the Mother of the Redeemer.