The Thursday of the Sixth Week of Easter is, in these United States, the Solemnity of the Ascension of the LORD. In some Diocese, this Solemnity has been moved to the Seventh Sunday of Easter, thus it becomes “Ascension Thursday Sunday” (or something…)
But we are blessed here, and Ascension Thursday is on Thursday. From the website of the Diocese of Portland, Maine:
“The Solemnity of the Ascension of the Lord occurs 40 days after Christ's resurrection and is the important, final piece of the paschal mystery, which began with Christ's passion and death. Because Christ ascended, we, as members of the Body of Christ, also look forward to ascending into heaven after our bodily resurrection.
On the solemnity, we are also reminded of our evangelizing mission. Before Christ ascends, he gives his disciples final instructions, telling them to await the arrival of the Holy Spirit and then "go into the whole world and proclaim the Gospel to every creature" (Mark 16:15). We will celebrate the descent of the Holy Spirit upon the apostles on Pentecost Sunday, May 20. During the days between the Solemnity of the Ascension and the Solemnity of Pentecost, we are called to intense prayer for the coming of the Holy Spirit. This time of prayer comprises a novena.
While some ecclesiastical provinces have transferred the celebration of the Ascension of the Lord to the Seventh Sunday of Easter, that is not the case in the province of Boston, of which the Diocese of Portland is a part. Here, the celebration remains on the proper Thursday and is a holy day of Obligation.”
A list of Mass times by city and town (also from the Diocesan website) is here.