Why is rose used in Advent, in the wreath and the vestments?
The short answer
Advent is a season of penance, so its color is violet. On the Third Sunday, Gaudete ("Rejoice") Sunday, the color brightens to rose to mark the joyful anticipation of Christmas.
Violet for penance
Because Advent is a time of penance, its ordinary liturgical color is violet.
Rose for joy
On the Third Sunday of Advent, called Gaudete Sunday (Latin for "Rejoice"), the color changes to rose to recognize the joyful anticipation of Christ's birth. Rose is a brightening of violet, and so symbolizes joy.