Also called days of precept, holy days are feasts of such importance in the liturgical calendar that attendance at Mass is required. The Code of Canon Law (cc. 1246-1248) discusses these, rightly beginning with Sunday, describing it as "the day on which the paschal mystery is celebrated in light of the apostolic tradition and is to be observed as the foremost day of obligation in the universal Church" (Can. 1246). It then lists the following to be observed: Christmas; Epiphany; Ascension; Corpus Christi; Mary, Mother of God; Immaculate Conception; Assumption; St. Joseph; Sts. Peter and Paul; and All Saints. This list is the same as that given in the 1917 code, with the feast of the Circumcision eliminated in favor of the restored title for January 1, Mary, Mother of God. The present code then states that "the conference of bishops can abolish certain holy days of obligation or transfer them to a Sunday with the prior approval of the Holy See" (Can. 1246). The United States bishops decided not to make the feasts of St. Joseph and Sts. Peter and Paul days of precept and transferred the Solemnities of the Epiphany and Corpus Christi to a Sunday.This year, presumably because All Saints' Day falls on the day after Sunday, our primary holy day, it is not a holy day of obligation in America. There's so much we could say about lowering the bar, and the primacy of convenience in the faith life of American Catholics, but let's just say that Masses will still be offered tomorrow and it would be a great day to attend and ask all the Saints in heaven to pray for you.
Thanks to the Catholic Dictionary (Our Sunday Visitor) for the definition. By way of endorsement, I'd like to say this is one of my favorite reference books on the Faith!