Unlike the Novus Ordo rite, the traditional rite celebrates the Immaculate Conception on its proper date, December 8, regardless of the day, meaning that it is not overridden by the Second Sunday of Advent. And this great feast should retain its proper day, as it is the official beginning of the story of our salvation. Mary is prepared to become the Mother of God from Her very conception, being preserved from the stain of Original Sin.
Enjoy this article from Catholic Restoration explaining this solemnity in depth.
O Mary, conceived without sin, pray for us who have recourse to Thee!