Dear Beverly:

This subject is called the Dormition of the Blessed Mother. (Dormition comes from the Latin dormire, meaning "to sleep." This is the same word that we get the word "Dormitory").

We really do not know for sure if Mary died and then was immediately assumed into heaven, or if she was assumed just before she would have died.

There have been people in the bible who were assumed into heaven without dying -- such as Elijah. There are various opinions about this.

All that we can definitive say is what the Church has officially declared as dogma by Pope Pius XII in 1950:

"Mary, the immaculate perpetually Virgin Mother of God, after the completion of her earthly life, was assumed body and soul into the glory of heaven." 

There are unverified stories about Mary's Dormition. For example, in an apocryphal collection of stories called Transitus Mariae (The Passage of Mary), attributed to Bishop St. Melito of Sardis (d. c. 200), Mary died in the presence of the apostles in Jerusalem, and then depending on the story, her body just disappeared, or was buried and then disappeared.

St. John Damascene (d. 749) also recorded an interesting story concerning the Assumption: "St. Juvenal, Bishop of Jerusalem, at the Council of Chalcedon (451), made known to the Emperor Marcian and Pulcheria, who wished to possess the body of the Mother of God, that Mary died in the presence of all the Apostles, but that her tomb, when opened, upon the request of St. Thomas, was found empty; wherefrom the Apostles concluded that the body was taken up to heaven."

These are not official explanations, but merely stories that circulated. The bottomline is that we do not know for sure whether Mary died or not. 

It is believed, however, that since she was full of grace, she remained preserved from the consequence of sin, namely corruption of the body after death and postponement of bodily happiness in heaven until the last day.

Thus, if she did literally die, she would have been assumed into heaven immediately before her body began to decay.

All we can really say is what the church says -- that she was assumed into heaven body and soul at the end of her earthly life.

God bless,
Bro. Ignatius Mary

P.S. By the way, I think that most scholars believe that Mary lived about 10-15 years after Pentecost.