December 8th is the feast of the patroness of the United States of America and it is also a holy day of Obligation. At the moment of Mary's conception, thanks to a special grace from God, she was free from original sin and retained this purity till she entered heaven.
The Mass times are as follows:
December 7:
Vigil Mass 5:00 p.m. Central - Twin Buttes
Vigil Mass 7:00 p.m. - Killdeer
December 8:
Mass 8:30 a.m. - Halliday
Mass 10:30 a.m. - Killdeer
And the Saturday evening Mass fulfills this obligation so long as one attends again on Sunday to fulfill the Lord's Day obligation.