1. The coming or arrival, especially of something extremely important: the advent of the computer.
a. The liturgical period preceding Christmas, beginning in Western churches on the fourth Sunday before Christmas and in Eastern churches in mid-November, and observed by many Christians as a season of prayer, fasting, and penitence.
b. Christianity The coming of Jesus at the Incarnation.
c. Christianity See Second Coming.
(Taken from: http://www.thefreedictionary.com/)