As we make plans to participate in the liturgy of the Church at Christmas, we are in a unique situation this year. December 25 will be on a Monday. Christmas is always a holy day of obligation, so Catholics need to participate in Masses both for the Fourth Sunday of Advent and for Christmas.
The Church does not offer a "two-for-one" option on Sunday evening, December 24, which means that one cannot attend just on Mass on Sunday to fulfill both the Sunday obligation and the Christmas obligation.
All Masses on Saturday evening, December 23, and Sunday, December 24, any time before 4:00 PM, will be for the Fourth Sunday of Advent. On Sunday, December 24, any Masses at 4:00 PM or later will be for the Vigil of the Nativity of the Lord (Christmas Eve) rather than for Sunday.
On the following weekend, the situation will be different. January 1 falls on a Monday this year. Since the Solemnity of Mary, Mother of God, takes place on a Monday, it is not a holy day of obligation in the U.S. in 2018. When an observance is not a holy day of obligation, there is no need for an anticipated Mass the evening before.
Therefore, on Saturday evening, December 30, and all day on Sunday, December 31, including the evening, all Masses will be for the Feast of the Holy Family. Masses celebrated any time on Monday, January 1, will be for the Solemnity of Mary, Mother of God. Although it is not a holy day of obligation this year, Mass is always a spiritual benefit.