The 40th day of Easter is Ascension Thursday, the day on which (the Bible tells us) Jesus Christ, having risen from the dead on Easter Sunday, ascended into Heaven.
Historically, the Ascension of Our Lord has been a Holy Day of Obligation--and it still is. However, in most parts of the United States today, Catholics will not attend Mass on Ascension Thursday--and they won't be violating the Precepts of the Church, which say that we have to assist at Mass on Holy Days, under pain of mortal sin. How can that be?
The answer is simple but confusing. The bishops of the United States, recognizing that attendance at Ascension Thursday Masses had dropped dramatically for years, petitioned the Vatican to allow them to transfer the celebration of Ascension to the following Sunday (the 43rd day of Easter). The Vatican agreed, but the decision was left up to each ecclesiastical province in the United States.
Several provinces continue to celebrate Ascension on Ascension Thursday, and you can find a list of them in Is Ascension a Holy Day of Obligation? Most provinces, however, transferred the celebration to the following Sunday, where it is still a Holy Day of Obligation. Since it coincides, however, with our Sunday Duty to assist at Mass, many people don't realize that they are, in fact, also fulfilling their duty to attend Mass on a Holy Day of Obligation.
(A stained-glass window of the Ascension of Our Lord in Saint Mary's Church, Painesville, OH. Photo © Scott P. Richert)