Q: This year Christmas falls on a Saturday. If we have a Saturday afternoon or evening Mass, what do we celebrate? Christmas or Holy Family?
A: Christmas. It outranks Holy Family.
But anyone who has already participated at a Christmas Mass and may use this later one in fulfillment of their obligation to attend Mass on Sunday.
Liturgical law is different from canon law. Canon law requires participation at a Sunday Mass, and liturgical law determines which Mass the priest celebrates on the Saturday evening before. So even though he’s celebrating Christmas on the evening of December 25, it fulfills the obligation to participate at Mass on Holy Family Sunday the 26th.