Holy Days of Obligation are not uniform throughout the universal church, because they were regulated by national Bishops' conferences even before Vatican II. Historically the United States has had less Holy Days of Obligation than other countries, especially the countries of Europe. However, I believe (though I'm not sure) that Canada only had 5 Holy Days before Vatican II. The United States has six.
In many countries of Europe days such as the Epiphany (January 6), St. Joseph's day during Lent (March 19th), Easter Monday, Pentecost Monday, and the Feast of Sts. Peter and Paul (June 29) are/were Holy Days of Obligation. I think in Ireland St. Patrick's Day is a Holy Day of Obligation, and there may be others for different countries, but those are the more common ones that are Holy Days for other countries.
Being a US resident, I attend Mass on all six Holy Days of Obligation observed in the US, regardless of whether the Modernists say you have to or not. I also try to attend Mass on those other days that are Holy Days on the Universal calendar, especially the Epiphany, even though it is not a Holy Day here in the US.