The holy days of obligation vary from country to country. The Church prescribes that all Sundays plus 10 solemnities are holy days of obligation. The solemnities in question are the Lord’s Nativity, Epiphany and Ascension plus Corpus Christi; of Our Lady: Mary, Mother of God, Assumption, and Immaculate Conception; of the saints: St Joseph, Ss Peter & Paul, and All Saints.
The bishops of each country can choose to remove the obligation from any of these. The Holy See asks that all countries keep the Lord’s Nativity and one of the solemnities of Our Lady as holy days of obligation. Each country’s decision has to be confirmed by the Holy See. I understand that if the Epiphany, Ascension or Corpus Christi are not kept as holy days of obligation they must be transferred to a Sunday.
I live in England and our bishops have decided that holy days of obligation in England are the four of Our Lord and kept on their proper days, the Assumption of Our Lady, and Ss Peter & Paul and All Saints. Here, Mary Mother of God, the Immaculate Conception and St Joseph are not holy days of obligation.
I do wish the Immaculate Conception was a holy day of obligation here. That was the dedication of my childhood parish and primary school. I think our neighbours in Scotland keep it as a holy day of obligation.