Basically, the observance of the Solemnity is transferred to Monday, December 9th since the Sunday of Advent takes precedence, but December 9th is not a Holy Day of obligation this year.
And here's why:
Taken from the USCCB Committee on Divine Worship Newsletter Volume XLIX February 2013
"In this year of 2013, December 8 – normally the Solemnity of the Immaculate Conception – falls on the Second Sunday of Advent. In accordance with no. 5 of the Universal Norms on the Liturgical Year and the Calendar, the observance of the Solemnity is transferred to Monday, December 9. Such a transfer is seen as a pastoral concession to the desire of the faithful to observe a beloved Feast even though it is impeded liturgically by a higher observance. The obligation of the faithful to attend Mass remains attached to the day itself (December 8), however, and so it does not transfer with the liturgical observance."