Mass for the Immaculate Conception of the Blessed Virgin Mary is celebrated on December 9th this year and it is not a Holy Day of Obligation. Masses on Monday, December 9th are at 6:45 am and 9:00 am.
The Immaculate Conception is celebrated on December 8th, which is a Sunday this year. The Sunday liturgy supersedes the Immaculate Conception Liturgy, so you technically fulfill your obligation to attend on December 8 by going to Sunday Mass. However, so as not to omit the observance of a Solemnity, the observance of the Immaculate Conception is transferred to the next available day (Monday in this case). So Mass on Monday will be for the Immaculate Conception of the BVM; you are encouraged to attend but you are not obligated (which is why we have just a normal Monday Mass schedule for the 9th).