This year is an interesting year with Christmas falling on a Monday. That means Christmas Eve is on a Sunday.
Father Z wrote a good explanation, and the original post can be found here
I like his point about going on Sunday and then on Christmas. Lets not have the “lets get it over with as quickly as possible” attitude.
In 1970, Paul VI approved a response to a “dubium” – those were the days – about double fulfillment by one Mass which bridges two days of precept: negative. We have two days of precept. That means two Masses.
There are the usual exceptions for the sick and shut in, etc. Also, parish priests can give individual dispensations for real necessities, as always.
No, we have to attend two Mass for the two obligations.
However, you could attend a Mass in the morning on Sunday, Christmas Eve Day, and fulfill that obligation, and then go again in the evening of the same day and fulfill the Christmas Day obligation.
Furthermore, if you go to Mass twice on Sunday (for the two obligations), you can also receive Holy Communion at both of those Masses. Canon Law says you can receive twice in a day, so long as the second time is in the context of Mass. So, if you went to two Masses in the morning on Christmas Eve Day, you could receive twice. You would have fulfilled your obligation. But if you went again, to a third Mass, in the evening of Sunday, Christmas Eve, you could fulfill your Christmas Day obligation, but you could not go to Communion a third time… unless you were in danger of death. Merry Christmas!
Or, just be a regular person who lets Sunday be Sunday and Christmas be Christmas and go on both Sunday and on Christmas. Right?
To make this more interesting, on Christmas, priests can celebrate three Masses and keep all three stipends! However, while the priest saying Masses can receive Communion three times on Christmas Day, you can’t. If you are at all three of Father’s Christmas Day Masses, you get Communion twice, not thrice.