Some “high feasts” do not rotate through a cycle of readings like Sundays do. (Some do, some do not.) I’m not 100%, but Christmas may be one of those. I know Easter Vigil is one such case. The readings are set and will be the same every year.
Because the lectionary (at least, according to the USCCB website) does not indicate a cycle for that Gospel, I would presume it is NOT variable, and thus always John.
