Some solemnities are so essential to our understanding of our redemption in Jesus Christ, and the duties of worship and praise we’re obliged to return to Him as a result, that they have been deemed as Holy Days of Obligation by the bishops. This means that the faithful are obliged to hear Mass on this day under pain of mortal sin—excepting those who have serious reason to be excused from this obligation, for example, in the case of illness.
Each and every Sunday is a Holy Day of Obligation, along with the following solemnities for those who live in the United States (unless the bishop transfers the celebration to the nearest Sunday—check with the diocese where you live).