When December 8th falls on a Sunday, the Solemnity of the Immaculate Conception is transferred to the following Monday, since it is outranked by the Sundays of Advent. It has been the practice int he U.S. to say that the obligation to attend Mass did not transfer with the feast. However, the Vatican has recently clarified that the day remains a Holy Day of Obligation even when it is transferred.
Therefore, Monday December 9th will be observed as a Holy Day of Obligation this year.