In my early Catholic days, I noticed the parish missalette contained a set of readings and prayers for a January 1 Mass for the World Day of Peace. I see the pope posts an annual message for the observance.
I don’t recall we ever used this Mass. I did know a priest from a religious order several years ago who preferred to celebrate a votive Mass for Peace on that day, rather than use the Marian readings. I see the day mentioned in liturgy planning guides–but no readings. As long as the feast is obligatory (and probably even if it isn’t) you use the readings given.
Take the purple chair and tell us how you would handle the balancing act of the 8th day of Christmas, the Marian feast, and world peace. Keep in mind that the Roman Missal seems to lean more heavily toward the Christmas observance, as the day’s prayers are taken from the Proper of Seasons, not the Proper of Saints.