A calendar is a system for naming periods of time, typically days. These names are known as calendar dates. Cycles in a calendar are often synchronised with the perceived motion of astronomical objects.
A calendar is also a physical device (often paper). This is the most common usage of the word.
As a subset, 'calendar' is also used to denote a list of particular set of planned events (for example, court calendar).
Advent Calendar - A web based traditional Advent calendar. Features animated doors, inspirational graphics, and Bible verses which are mostly from Psalms.
Ann's Advent Calendar - Each day provides a holiday image, musical selection, and a prayer in the Catholic tradition.
Celebrate Meeting Jesus - On-line advent calendar with a reflection on people in the New Testament for each day.
Mary Page Advent Calendar - A numbered nativity scene provides places to click to bring up meditations and Bible history; from the Marian Library at the International Marian Research Institute. [Roman Catholic]