Ascension Thursday is the fortieth day after Easter Sunday, commemorating the Ascension of Christ into heaven. Tradition points to Mount Olivet, near Bethany, as the place where Christ left the earth. The ecumenical feast falls on a Thursday. Forty days after Easter and ten before the feast of Pentecost.
Certain customs were connected with the liturgy of this feast of which I find very interesting, such as, "the blessing of beans and grapes after the Commemoration of the Dead in the Canon of the Mass, the blessing of a candle, the wearing of mitres by deacon and sub deacon, the extinction of the paschal candle, and triumphal processions with torches and banners outside the churches to commemorate the entry of Christ into heaven."
Why do most provinces celebrate Ascension Thursday on Sunday?
The simplest explanation is out of the convenience of the priests and the congregation. "Since Catholics are already obligated to attend Mass on Sunday, most people don't realize that Ascension is still a Holy Day of Obligation--they think they're in church just for Sunday Mass."Each ecclesiastical province in the United States is allowed to decide whether to transfer the celebration of The Ascension. (An ecclesiastical province is basically, one large archdiocese and the dioceses that are historically associated with it. Generally, in the United States, there's one ecclesiastical province per state, with a few exceptions for historical reasons.) All of the provinces have chosen to transfer the celebration except for Boston, Hartford, New York, Newark, and Philadelphia. Each of those is one of the oldest Catholic provinces in the United States, which may explain why they've chosen to stick with tradition.
Attendance at Ascension Thursday Masses had been falling for years before the bishops of the United States petitioned the Vatican to allow the celebration to be transferred to the following Sunday.
So there you have it... The reason for changing the dates of our Lord's Ascension into Heaven occurred out of laziness from the bishops and congregations of the United States.