On November 1, 1950, Pope Pius XII solemnly declared:

By the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.

This dogma of the Assumption of Mary to heaven body and soul is celebrated August 15th in our Roman Catholic liturgical calendar.  This belief is shared by Christians of the Roman Catholic Church, the Eastern Catholic Churches, the Eastern Orthodox Churches and the Anglican Communion.  

This event is known in the Eastern and Oriental Orthodox Churches as the "Dormition of Mary" or "falling asleep of the Virgin."  It has long been traditionally supported by the Fathers of the Church and infallibly supported by popes.  It is a Catholic Holy Day of Obligation in many countries.

Mary's role in the life of the Church does not limit itself to the piety and devotions we accord her.  According to the documents of the Council, most specifically Chapter VIII of Lumen Gentium , Mary typifies the Church.  

63. By reason of the gift and role of divine maternity, by which she is united with her Son, the Redeemer, and with His singular graces and functions, the Blessed Virgin is also intimately united with the Church. As Saint Ambrose taught, the Mother of God is a type of the Church in the order of faith, charity and perfect union with Christ.18* For in the mystery of the Church, which is itself rightly called mother and virgin, the Blessed Virgin stands out in eminent and singular fashion as exemplar both of virgin and mother. 19* By her belief and obedience, not knowing man but overshadowed by the Holy Spirit, as the new Eve she brought forth on earth the very Son of the Father, showing an undefiled faith, not in the word of the ancient serpent, but in that of God's messenger. The Son whom she brought forth is He whom God placed as the first-born among many brethren,299 namely the faithful, in whose birth and education she cooperates with a maternal love.

We celebrate the Assumption of Mary because in this event God fulfills the promise made by Jesus at the Last Supper : "If I go and prepare a place for you, I will come again, and will receive you to myself; that where I am, you may be there also."  It is the anticipation of our own going to heaven.  Mary, the first of the faithful disciples, was conceived without original sin in preparation for her singular and  unrepeatable role of being the Mother of God. It makes sense that she would not suffer death, a penalty of sin, but would instead be taken body and soul in heaven,  to be reunited with her Son.  On the other hand, we, because of our original sin, would have to die.  But our faith in the Resurrection tells us that death is not the end but only the beginning of life in God.  Our Creed tells us that 'we believe in the resurrection of the body and life everlasting."  This belief is confirmed in the Assumption of Mary.

This is one of my favorite passages in the New Testament which is also inspirational in my religious life.  It summarizes the reality of the Solemnity of Assumption and the life of Our Lady:  "If you have been raised with Christ, seek the things that are above, where Christ is seated at the right hand of God.  Set your minds on things that are above, not on things that are on earth.  For you have died, and your life is hid with Christ in God.  When Christ who is our life appears, then you also will appear with Him in glory." (Col 3:1-4)