One of the things that makes the Roman Catholic Church distinct from other Christian denominations is that we have days throughout the year– in addition to Sundays– when we’re obligated to attend Mass.
The six Holy Days are: Solemnity of the Immaculate Conception (December 8th), The Nativity of the Lord (Christmas), Solemnity of Mary the Mother of God (January 1st), Ascension of the Lord (the date moves with Easter), Solemnity of the Assumption (August 15th), and the Solemnity of All Saints (November 1st). Notice that Ash Wednesday isn’t on the list, even though it’s one of the busiest days of the year for most parishes.
There is a wrinkle. When the Assumption, All Saints, or the Solemnity of Mary falls on a Monday or Saturday, there is no Mass obligation. This year, for example, the Assumption falls on Monday, August 15, so there is no obligation to attend Mass, although Catholics are encouraged to celebrate this beautiful feast. The Immaculate Conception, as the Patronal Feast of the United States, is always a day of obligation, except when it falls on a Sunday, in which case it’s moved to the ninth, but without the obligation to attend Mass.
Finally, since 1992 the Feast of the Ascension is celebrated on the 7th Sunday of Easter in most, but not all, parts of the U.S.
Confused? Check the website or bulletin of your local parish when a Holy Day is approaching.