Note: this lazy practice is localized to the Catholic Church in America. The rest of the Church still observes the Holy Days.
And the UK........perhaps not surprisingly.
It probably started here but, alas, the practice of changing Holy Days to the nearest Sunday is pretty widespread, even in places like Spain. I believe it’s the option of the local bishop, however, and while the national bishops’ council can recommend putting all the Holy Days on Sundays, the local bishop is the one who makes that decision.
We need to go back to the Holy Days as they were. They were all there for a reason (a reference to the specific time in the Gospel accounts, and then the symbolic understanding of this by the Church over the centuries). Putting it on Sunday completely loses that.
Being stuck in my old missals and Rosary booklets, I had no idea they were changed.