In my diocese, January 1, the Solemnity of Mary, Mother of God, was not a holy day of obligation. But in another U.S. diocese, it was. Could you please explain?
In accord with the provisions of canon 1246 §2 of the Code of Canon Law, the USCCB has ruled: "Whenever January 1, the solemnity of Mary, Mother of God, or August 15, the solemnity of the Assumption, or November 1, the solemnity of All Saints, falls on a Saturday or on a Monday, the precept to attend Mass is abrogated" (www.usccb.org).
This covers the bulk of instances in which you will find holy days of obligation dispensed for Latin-rite Catholics in the United States. However, there is one more provision of canon law to keep in mind: "Whenever he judges that it contributes to their spiritual welfare, the diocesan bishop can dispense the faithful from disciplinary laws, both universal laws and those particular laws made by the supreme ecclesiastical authority, for his territory or his subjects" (CIC 87 §1).
This means that an individual bishop can dispense the obligation of a holy day of obligation for his diocese. That is why in some U.S. dioceses January 1 is celebrated as a holy day of obligation, and in others it is not.