Many Catholics in the United States are surprised when I tell them that, in the Latin Rite of the Catholic Church (the rite to which most of us belong), there are ten Holy Days of Obligation every year. But it's true: You can find all ten listed in Canon 1246 of the 1983 Code of Canon Law.
When you look at that list, however, you'll see four days that Catholics here in the United States do not not think of as Holy Days of Obligation (and I'm not even counting the Ascension of Our Lord, which is transferred, in most dioceses in the United States, to the Sunday following Ascension Thursday).
Each bishops' conference can, with the approval of the Vatican, transfer the celebration of certain Holy Days of Obligation to Sunday (when all Catholics are obliged to attend Mass anyway) or abolish the obligation altogether. The bishops of the United States have transferred two Holy Days of Obligation (in addition to Ascension) and abolished two.
You can find out which Holy Days of Obligation have been affected by comparing these two lists: Holy Days of Obligation in the Latin Rite of the Catholic Church and Holy Days of Obligation in the United States.
The bishops' conference of a country can also add Holy Days of Obligation to the calendar, not just subtract them. Can you guess some of the feasts that have been added as Holy Days of Obligation, and the countries in which they have been added? Leave your guesses in the comments below!