As happened about six or seven years ago when Christmas day last fell on a Monday, people have understandably asked this question: if I normally go to Mass on a Sunday evening, does that satisfy at once the obligation to go to Mass both on the Sunday and at Christmas? Guidance given by the Liturgy Office of the Bishops Conference makes it clear that the answer is ‘No!’
The statement says: ‘The primary purpose of a holy day of obligation is to call the faithful to celebrate particular dimensions of the mysteries of Christ and His Church. Since obligation is attached to specific mysteries being celebrated, it is not possible to fulfil two obligations in one celebration.’
This means that you are expected to go to Mass on the Fourth Sunday of Advent and to Mass on Christmas Eve or Christmas Day. It has been made clear that you cannot do these things by attending one single Mass on the evening of Christmas Eve. You could do so by attending two such Masses (and this parish you can do this as we have three early evening Masses on Christmas Eve) but it would be much better to go to an Advent Mass on the Saturday evening or Sunday morning and then a Christmas Mass on Sunday evening or Monday morning.