Ascension of the Lord

Question: When is Ascension Thursday?

Answer: 40 days after Easter. 
   But only in the ecclesial provinces of :

     Boston

     Hartford

     New York

     Newark

     Philadelphia

     and the state of Nebraska.

In all other dioceses of the United States, the celebration of the Solemnity of the Ascension is transferred to the following Sunday.

Including here in the Archdiocese of Portland. 

This change was approved by Pope Saint John Paul II in 1999, subject to approval by the bishops in each diocese. Most dioceses voted for the change except the above-mentioned ecclesial provinces.

If you are really interested in the Canon Law, see this link from the website of the United States Catholic Bishops:

     Decree on transfer of the date of the Solemnity of the Ascension