August 15th is the Feast of the Assumption of Mary, when we celebrate the Blessed Mother of Jesus being assumed body and soul into heaven. It is a beautiful, tender moment and a lovely feast for a lovely lady.
If you are interested in the basis for this feast please read these two sources (there are lots online, these are just two):
The Assumption of the Blessed Virgin Mary by Marcellino D'Ambrosio and
Assumption of Mary into Heaven by Msgr. Charles Pope at the Archdiocese of Washington
Update: You should find the Women for Faith & Family Assumption page to be very informative also. It includes the history, readings for the day, prayers and devotions and more...
Also: Fr. Z's wonderful Meditations of the Assumption as part of his excellent Patristic Rosary Project
Image: The Assumption of the Virgin, Peter Paul Rubens, National Gallery of Art, Washington, D.C.
More Info: Shafe