The Vatican agreed. Today, only the ecclesiastical provinces of Boston, Hartford, New York, Newark, Philadelphia, and Omaha (the state of Nebraska) continue to celebrate the Ascension of Our Lord on Thursday. The faithful in those provinces (an ecclesiastical province is basically one large archdiocese and the dioceses that are historically associated with it) are required, under the Precepts of the Church, to attend Mass on Ascension Thursday.
In the rest of the dioceses of the United States, the celebration of the Ascension has been transferred to the following Sunday (the 43rd day after Easter). That does not mean, however, that the Ascension is no longer a Holy Day of Obligation in those dioceses. Under the Precepts of the Church, every Sunday is a Holy Day of Obligation.
Because attendance at Mass on a holy day is obligatory (under pain of mortal sin), if you have any doubts about whether you live in one of the provinces that celebrates the Ascension on Thursday, you should check with your parish priest or diocesan office.
For the date of the Ascension (both the Thursday celebration and the Sunday transferred celebration) in this and future years, see When Is Ascension?