Currently the local bishops are responsible for assigning the day when the Ascension is celebrated.
Just one verse each day.
After the Resurrection on Easter Sunday, Jesus did not immediately go back to Heaven. Luke in the Acts of the Apostles explains how, “To [the apostles, Jesus] presented himself alive after his passion by many proofs, appearing to them during forty days, and speaking of the kingdom of God” (Acts 1:3).
This fact is repeated later on in Acts, when Paul confirms, “God raised [Jesus] from the dead; and for many days he appeared to those who came up with him from Galilee to Jerusalem, who are now his witnesses to the people” (Acts 13:30).
Based on these passages, the Church celebrated the Solemnity of the Ascension, commemorating the day when Jesus returned to his Father in Heaven, exactly 40 days after Easter Sunday, marking it as a day when the faithful are obligated to attend Mass. Counting 40 days after Easter means the feast of the Ascension occurs on a Thursday.
Theologians have pointed out how Thursday is symbolically tied to the celebration of the Last Supper, thus making the Ascension a Eucharistic feast, fulfilling Jesus’ promise, “I am with you always, to the close of the age” (Matthew 28:20). This makes the feast of the Ascension a day of rejoicing, even though for the apostles it must have been mixed with grief.
For centuries, the Ascension was celebrated exactly 40 days after Easter Sunday. However, recent practice has sought to make it easier for the faithful to attend Mass on this important day, and individual bishops’ conferences are now responsible for assigning the date for the celebration of the Ascension. The Code of Canon Law stipulates, “the conference of bishops can abolish certain holy days of obligation or transfer them to a Sunday with prior approval of the Apostolic See” (1246, §2).
As a result, the US bishops’ conference explains in its liturgical calendar, “In several ecclesiastical provinces of the United States of America, the Solemnity of the Ascension of the Lord is transferred from Thursday, to the following Sunday.”
The reason behind this transfer in parts of the United States is that the Ascension is not connected to any civil celebration (thus no time off work) and proper participation in the feast can be difficult for many families. Essentially, most people would have difficulties attending Mass on a Thursday (or Wednesday evening) since there is no civil holiday.
Other holy days of obligation — like Christmas and New Years Day — do have civil celebrations and allow for greater participation among the faithful.
Thus for these pastoral reasons, almost all regions in the United States have moved the celebration of the Ascension from Thursday to Sunday, to make it more accessible to the lay faithful. Check your parish bulletin or diocesan website for information on the feast in your area.