The first precept ("You shall attend Mass on Sundays and holy days of obligation and rest from servile labor") requires the faithful to sanctify the day commemorating the Resurrection of the Lord as well as the principal liturgical feasts honoring the mysteries of the Lord, the Blessed Virgin Mary, and the saints; in the first place, by participating in the Eucharistic celebration, in which the Christian community is gathered, and by resting from those works and activities which could impede such a sanctification of these days.(Catechism of the Catholic Church 2042)
Thursday, August 15th is a Holy Day of obligation - the Solemnity of the Assumption of the Blessed Virgin Mary into Heaven. On this day, every Catholic is obligated to attend Mass (unless there is a grave cause which prevents him). It is a mortal sin to purposefully skip a holy day.
This feast is a time to recall that our Lady has not left us as orphans, but continues to watch over and guide her faithful children to the heavenly glory she now enjoys. As she was assumed body and soul into the glories of heaven, we look forward to the day when our mortal bodies will be transformed to the likeness of Christ's glorified body.
At Corpus Christi, there will be a vigil Mass on Wednesday eve (which counts for the Holy Day), as well as two Masses on Thursday, August 15th.
Wednesday, August 14th - 5:30pm (vigil)
Thursday, August 15th - 6:30am and 5:30pm