Holy Days of Obligation

What are "Holy Days of Obligation"?  
They are special dates of the Church calendar that mark significant Solemnities of our Catholic faith. Since they don't always fall on Sunday, except once every 7 years, the Church obliges us to celebrate them on whatever day of the week they do occur.

We celebrate 5 Holy Days of Obligation each year: 

January 1 - Mary, Mother of God

August 15 - The Assumption of Mary

November 1 - All Saints Day

December 8 - Immaculate Conception of Mary

December 25 - The Nativity of the Lord

All Catholics are bound to attend Mass on these days (or a vigil Mass the night before). The duty here is just like the duty to attend Mass every Sunday. Missing Mass for any reason other than sickness or injury requires a person to go to Confession before they can again receive Holy Communion.