August 15 is the Solemnity of the Assumption of Mary into heaven.
It is one of the Holy Days of Obligation in the Catholic Church.
Holy Day of Obligation means that it is a day that should be given the reverence worthy of a Sunday.
Holy days of Obligation are days on which the faithful are expected to attend Mass, and engage in rest from work and recreation, according to the Third Commandment.
Can. 1246. §1. Sunday, on which by apostolic tradition the paschal mystery is celebrated, must be observed in the universal Church as the primordial holy day of obligation.
The following days must also be observed: the Nativity of our Lord Jesus Christ, the Epiphany, the Ascension, the Body and Blood of Christ, Holy Mary the Mother of God, her Immaculate Conception, her Assumption, Saint Joseph, Saint Peter and Saint Paul the Apostles, and All Saints. §2.
With the prior approval of the Apostolic See, however, the conference of bishops can suppress some of the holy days of obligation or transfer them to a Sunday.
—Friar. Cornelius Livingstone Ikemba