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.
For example, on December 13, 1991, the USCCB made the following general decree concerning holy days of obligation for Latin Rite Catholics:
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.