Advent season ('Advent') starts four full weeks prior to Christmas. On four subsequent Sundays the family joins around the traditional advent wreath with four candles ('Adventkranz'). Each Sunday one more candle is lit to mark the advent of Christmas. Often this time is used for singing Christmas carols and generally getting ready for Christmas. This might include reading stories, drinking tea and getting a first taste of delicious Christmas cookies. (Strictly speaking 'Advent' is a season of fasting but it is a save bet that at least as many people nibble cookies as manage to refrain.)
An advent calendar ('Adventkalender') features 24 little bags or windows that you can open one by one starting at the 1st of December. The bags contain little presents; the windows contain either images or chocolate. When you open the last window/bag Christmas Day has arrived.
Children are asked to compose a list of wishes addressed to the 'Christkindl', which fetches all lists from the windowsill and leaves a small surprise present. Speaking of the 'Christkindl': Christkindlmärkte are another fascinating aspect of advent season.