According to the liturgical calendar of the 1962 Missal still in force, this Sunday, December 8, is the feast of the Immaculate Conception of the Blessed Virgin Mary.
The Novus Ordo has transfered its feast to Monday while declaring there is no obligation to hear Mass on the transfered holy day.
"The traditional Latin Mass, however, follows the calendar and rubrics in place for 1962, as specified in #28 of Universae Ecclesiae. To that end, when the first class feast of the Immaculate Conception falls on the same day as the first class Second Sunday of Advent, the feast of the Immaculate Conception is observed, and the Second Sunday of Advent is commemorated."
[Hat tip to Kenneth J. Wolfe at Rorate Caeli, 12/6/2013]