The Immaculate Conception is a solemnity in the United States, a solemnity which is never skipped.
The Immaculate Conception is the Patroness of the United States. Her feast day is a holy day of obligation. Most parishes have a vigil Mass on the evening before, as well as the holy day Mass on the day itself. This year the feast falls on Saturday - so one must attend a Mass for the Immaculate Conception anytime from Friday evening (vespers) through Saturday, the day of the feast. In most parishes, there is also a vigil Mass for Sunday. Normally, and according to contemporary custom, the vigil Mass for Sunday does not satisfy for the feast day obligation. Norms in the U.S. are commonly understood to be that Catholics are obliged to attend one Mass for the Immaculate Conception, and another Mass to fulfill the Sunday obligation when such a juxtaposition occurs. This often happens for Christmas as well, unless Christmas Day falls on a Sunday. If you remain confused, ask your chancery or your parish priest. There may be pastoral provisions in cases of hardship or ignorance.
You really do not have to consult a canon lawyer or a priest blogger for such simple instructions, much less seek their interpretation or mere opinion as regards norms set up by the bishops conference or local ordinary of your area. The Bishops make the rules. In my parish we were instructed the vigil Mass for Sunday does not satisfy for the feast day Mass, the parish priest was instructing us in accord with the teaching of the local ordinary.
Always call your chancery or parish priest for the the rules and regulations which apply in your diocese, or when you are traveling. Really. A priest-spokesman for the chancery here told me that is why they are there.