Holy Days
Holy Days of Obligatation are important days in the Catholic Church that honor special events or special people. Throughout the year, there are specific feasts of Our Lord, Our Lady, and the saints that we observe a "solemnities." On these days,, Catholics are obliged to attend a holy day Mass on the day itself or a vigil Mass the evening before. That's why thse special days are called Holy Days of Obligation.
Holy Days links
Holy Days of Obligation (OSV's Encyclopedia of Catholic Doctrine)
Holy Days of Obligation (United States Conference of Catholic Bishops)
Holy Days of Obligation (Catholic Online)
Holy Days of Obligation (EWTN)
Sunday Mass and Holy Day Obligation (EWTN)
Holy Days of Obligation (New Advent)
Dies Domini (Apostolic Letter of the Holy Father John Paul II)