The United States Catholic Conference of Bishops (USCCB) have communicated that according to  Canon 1246.2 in the Code of Canon Law, when January 1 falls on a Saturday or Monday, January 1 (the Solemnity of Mary, Mother God) is not a Holy Day of Obligation.

However, there is more to the story. Another Canon ~ 87.1 mentions that an individual bishop can make a dispensation for the holy day of obligation for their 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.