Tomorrow, Thursday, May 25, 2017, is a Holy Day of Obligation for SOME Catholics.

Scott P. Richert from ThroughtCo. can shed more light on my above comment:

CELEBRATING ASCENSION THURSDAY

The Vatican agreed. Today, 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.

CELEBRATING ASCENSION ON SUNDAY

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). That does not mean, however, that the Ascension is no longer a Holy Day of Obligation in those dioceses. Under the Precepts of the Church, every Sunday is a Holy Day of Obligation.

Because attendance at Mass on a holy day is obligatory (under pain of mortal sin), if you have any doubts about whether you live in one of the provinces that celebrates the Ascension on Thursday, you should check with your parish priest or diocesan office.

For the date of the Ascension (both the Thursday celebration and the Sunday transferred celebration) in this and future years, see When Is Ascension?

Kudos to Christopher Ferraro for the above graphic.

A wonderful way to reflect on The Ascension of our Lord, is to read, pray and ponder the Catechism:

ARTICLE 6
“HE ASCENDED INTO HEAVEN AND IS SEATED AT THE RIGHT HAND OF THE FATHER”

From the Catechism of the Catholic Church:

660 The veiled character of the glory of the Risen One during this time is intimated in his mysterious words to Mary Magdalene: “I have not yet ascended to the Father; but go to my brethren and say to them, I am ascending to my Father and your Father, to my God and your God.”⁵³⁷ This indicates a difference in manifestation between the glory of the risen Christ and that of the Christ exalted to the Father’s right hand, a transition marked by the historical and transcendent event of the Ascension.