When it is capitalized, Advent means the month (or four weeks) before Christmas, it is a season of the Christian year for some churches. Also within the Christian faith, Advent means the arrival of Jesus Christ, either when he was born or his Second Coming. An Advent calendar is a countdown during the month of Advent until Christmas. Usually there is a candy or present for each day.
When advent is not capitalized means the coming of an important or note-worthy person or object.
The Advent calendar is sometimes spelled with a lower-case a, which makes sense that it is about the arrival of an important date. However, it is really a countdown of the time of Advent and should be capitalized.
The word comes from the Latin adventus or arrival.
An Adventist is a person who belongs to a sect of Christianity which places great focus on the Second Coming of Christ. Their believe system is called Adventism.
Baba who stated this in Abuja on the occasion of 2013 NIPOST Headquarters Merit Awards, debunked the widely held belief that the advent of ICT has become a threat to the existence of NIPOST, as some many Nigerians do not write letters and dispatch them via the post offices. [All Africa]
A cursory search corroborates this: The company offers several different themed calendars for sale this year, including a “friends” Advent calendar — which is pink and purple and I assume is targeting small human girls; a “city” Advent calendar; and, brand upon brand, a “Star Wars” Advent calendar. [Washington Post]
During the “Second Great Awakening” new Protestant denominations emerged such as Adventism, the Restoration Movement, and groups such as Jehovah’s Witnesses and Mormonism. [Morung Express]