The Ascension of the Lord — Sunday, May 17th — Not Thursday, May 14th
Although our Church Calendar lists Thursday, May 14th, as the Ascension of the Lord, here in the Diocese of San Diego (and in most dioceses in the United States), the Ascension will be celebrated next Sunday, May 17th.
So Thursday, May 14th, is not a Holy Day of Obligation here in the Diocese of San Diego.
Now, 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).
Below is an explanation of the fact that in different dioceses, the Ascension of the Lord is celebrated either on Thursday, May 14th, or on Sunday, May 17th.
Determining the date of Ascension is also complicated by the fact that, in many dioceses of the United States (or, more accurately, many ecclesiastical provinces), the celebration of Ascension has been transferred from Ascension Thursday (40 days after Easter) to the following Sunday (43 days after Easter). Since Ascension is a Holy Day of Obligation, it is important for Catholics to know on which date Ascension will be celebrated in their particular diocese. (See Is Ascension a Holy Day of Obligation? to find out which ecclesiastical provinces continue to celebrate Ascension on Ascension Thursday, and which have transferred the celebration to the following Sunday.)
For more information, click here: http://catholicism.about.com/od/When-Is/f/When-Is-Ascension.htm