Catholics are obliged to attend Mass twice between the evening of Friday, 12/24, and the evening of Sunday 12/26. One of those times needs to happen before Sunday. The other of those times has to happen Saturday evening or after. Going to Mass Saturday evening can satisfy either obligation, but not both at the same time. As ever, it is the attendance at Mass that satisfies the obligation, not the specific liturgy of that Mass.
In the chart below, the yellow boxes indicate when the first obligation may be fulfilled. The blue boxes indicate when the second obligation may be fulfilled. The content of the boxes indicates which liturgical celebration is to take place. Please note that the Saturday evening celebration, even if in fulfillment of the Sunday obligation, must be the celebration of Christmas Day and not the Holy Family.