Advent is the Christian festival preparing for Christmas. It is celebrated with candle lighting, wreaths and special services. Advent lasts until 24th December. Advent calendars usually start from 1st December but advent itself is the nearest Sunday to the 1st December.

Religious in Origin

Advent calendars were originally religious in origin but now there are many which are more secular. They make nice birthday presents for those with birthdays in December. 

An Advent calendar is a special calendar used to count the days of Advent in anticipation of Christmas. Since the date of the First Sunday of Advent varies, falling between November 27 and December 3 inclusive, many Advent calendars, especially those that are reusable, often begin on December 1, although those that are produced for a specific year start from the First Sunday of Advent and thus include the last few days of November that begin the liturgical season of Advent. The Advent calendar was first used by German Lutherans in the 19th and 20th centuries, and has since then spread to other Christian denominations. - Wikipedia