"White - Double - 1st Class - Immaculate Conception of the Blessed Virgin Mary"
The feast of the Immaculate Conception of the Blessed Virgin Mary was instituted to solemnize the exalted dignity bestowed upon her through a special grace of God, and in behalf of the merits of Chirst - a dignity and choice that preserved her from the least stain of original sin.
We must look kto the Orient for the first indications of this feast . In the Eastern Church it was observed as early as the seventh century, and in the Western Church it became known in the ninth century. Pope Sixtus IV introduced it (1476) into the Roman Church; Pope Gregory XV gave to it (1622) the title, "conception B.V.M. Immaculatae"; Innocent XII raised it (1693) to a feast of the 2nd class with an octave; Clement XI declared it (1708) a universal holyday; Pius IX gave it (1863) new Beviary hours and a new Mass; Leo XIII raised it (1879) to a feast of the first class with a vigil. The choice of December 8th was determined and approved solely in accordance with its tradition. - SM"
In the US, thDec. 8th is a holy day of obligation. She is the patron Saint of the United States.
I'm sure Roger Cardinal Mahoney is disappointed he can't give this feast a pass, as he will do with Jan 1st in 2010 (once again for at least the 3rd year running.)
Note that in the older calendars this feast used to be an Octave. no Octave for the '62 Missal.