Declared dogma in 1854 by Pope Pius IX, this Holy Day of Obligation commemorates St. Mary being concieved without the stain of original sin. Being filled with grace, unbroken by sin, Mary was preserved from the moment of her conception to be the mother of the Savior, Jesus Christ. The Immaculate Conception is the patroness of the United States of America. The National Basilica is dedicated to her in Washington D.C.
Pope Pius X, on the 50th anniversary of this feast, wrote his encyclical Ad Diem Illum Laetissimum, a worthy read and very loving document which can be found on the Vatican website. From the encyclical:
"For can anyone fail to see that there is no surer or more direct road than by Mary for uniting all mankind in Christ and obtaining through Him the perfect adoption of sons, that we may be holy and immaculate in the sight of God?"
Read more about the Immaculate Conception
How do we explain the Immaculate Conception to non-Catholics?
An article by Tim Staples regarding the importance and meaning of the Immaculate Conception