Whenever the solemnity of Mary's Assumption into Heaven falls on a Monday, it is no longer a holy day of obligation in the United States.
Every year the Catholic Church honors the Assumption of the Blessed Virgin Mary into Heaven on August 15. For many Roman Catholics, it is a holy day of obligation.
However, local bishops have the power to suppress the obligation if there is a pastoral need.
Whenever January 1, the solemnity of Mary, Mother of God, or August 15, the solemnity of the Assumption, or November 1, the solemnity of All Saints, falls on a Saturday or on a Monday, the precept to attend Mass is abrogated.
This means that there is no obligation in the United States to attend Mass if the solemnity of the Assumption falls on a Monday. Other countries may have different dispensations, and it is possible that August 15 remains a holy day of obligation. It is important to consult your local parish priest, especially if you live outside of the United States.
At the same time, this does not mean that the Assumption is no longer celebrated. Many parishes will still have one or two Masses on August 15 to celebrate the solemnity, though Catholics are not obliged to attend.