Is the Ascension a holy day of obligation?

Yes, the solemnity of the Ascension is a holy day of obligation, but the exact day of its celebration depends on your local bishop.

One of the principal feasts of the liturgical year is the solemnity of the Ascension of the Lord, commemorating when Jesus ascended into Heaven.

The event is narrated in the Acts of the Apostles:

So when they had come together, they asked him, “Lord, will you at this time restore the kingdom to Israel?” He said to them, “It is not for you to know times or seasons which the Father has fixed by his own authority. But you shall receive power when the Holy Spirit has come upon you; and you shall be my witnesses in Jerusalem and in all Judea and Samar′ia and to the end of the earth.” And when he had said this, as they were looking on, he was lifted up, and a cloud took him out of their sight.
Acts 1:6-9

When is the Ascension celebrated?

The Catholic Church celebrates the Ascension 40 days after Easter Sunday, and that day normally falls on a Thursday.

This correlates with St. Luke’s account in the Acts of the Apostles, where he states, “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).

However, to make it easier for the faithful to attend Mass on this important day, individual bishops’ conferences are responsible for assigning the date for the celebration of the Ascension and can transfer it to the nearest Sunday.

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).

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.”

Ascension on Thursday

There are a handful of Ecclesiastical Provinces in the United States (as well as other parts of the world) that have chosen to continue to observe the Solemnity of the Ascension on Thursday, such as Boston, Hartford, New York, Omaha, and Philadelphia. The decision impacts 9 different U.S. states overall.

Also, the Dioceses of England and Wales moved the Ascension to Sunday in 2007, but then in 2018 moved it back to Thursday.

Whenever it is celebrated, the Ascension is a holy day of obligation.