The Code of Canon Law of the Catholic Church says that there are nine holy days of obligation other than Sundays, but it also says that the local conference of bishops may suppress some of them or transfer them to a Sunday.
Bishops in the United States have suppressed the feast of Saint Joseph and the feast of Saints Peter and Paul as holy days of obligation. In addition, the feast of Epiphany is always celebrated on a Sunday in the U.S., and in the Diocese of La Crosse, the Ascension of Jesus is always on the 7th Sunday after Easter.
This leaves us with five holy days of obligation outside of Sundays. The following are holy days of obligation in the U.S.
Solemnity of Mary - January 1*
The Assumption of Mary - August 15*
Solemnity of All Saints - November 1*
Immaculate Conception of the Blessed Virgin Mary - December 8
Christmas - December 25
*Not a Holy Day of Obligation if it falls on a Saturday or Monday.