The number of Days of Obligation vary from year to year, because the precept to attend Mass is lifted (abrogated) if any of the following days:
falls on a Saturday or on a Monday. The faithful, however, are encouraged to attend Mass on those days. This was decided by the National Conference of Catholic Bishops of the United States of America on December 13, 1991 (Canon 1246) and is effective as of January 1, 1993.
Ascension Thursday is a Holy Day of Obligation in the entirety of the following States: Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, and Nebraska.
In Hawaii since 1992 the only observed Days of Holy Obligation (except Sundays) are Feast of the Immaculate Conception and Christmas.