Dustin Thomas says his two young children look forward to Thanksgiving each year, not so much for the turkey and stuffing, but rather because that's when his mother hands out their Advent calendars.

"Since Thanksgiving is so close to Advent, she gives one calendar to every grandchild that day, but always reminds them to wait until Dec. 1 to open the first door -- and to open just one a day," he says. "That's because one year, when we weren't paying attention, our two kids randomly opened several of the doors and ate a bunch of the chocolate. So, of course, that defeated the purpose."

For the Thomas family and countless others since its origins in the fourth century, Advent is observed as the four Sundays preceding Christmas. It begins on the Sunday closest to the feast day of St. Andrew the Apostle -- which is Nov. 30 -- and continues for the next three Sundays, ending on Christmas Eve. The word Advent is derived from the Latin word adventus, which means coming. Although Advent has long been known as a time for conversion, today the season is commonly associated with the anticipation of celebrating the anniversary of Christ's birth.

Like many other aspects of our modern Christmas celebrations, the Advent calendar is of German origin, says historian and blogger Alex Wakelam. Modern Advent calendars often include paper doors that open to reveal a Bible verse, an image or a piece of chocolate. That tradition dates to the mid-19th century, when German Protestants made chalk marks on doors or lit candles to count down to Christmas. Later, following the newer tradition of adding new devotionals with each day, the first-known handmade wooden Advent calendar was produced in 1851, says Wakelam.

The first printed Advent calendars were introduced in the early 1900s. By the 1920s, Gerhard Lang -- known as the father of the modern Advent calendar -- modified his calendar to include the tiny doors that we see today. Unfortunately, Lang's business was closed when cardboard rationing took place in the 1930s. At the end of World War II, Richard Sellmer began printing them again.

Calendars with chocolate, like those that the Thomas family uses, began to appear in the late 1950s, says Wakelam, and they soon gained popularity around the world. They usually display a colorful Nativity or holiday scene with numbered windows scattered about.

However, not all Advent calendars are premade. For example, some religion and Sunday school teachers urge children to make their own calendar from cardboard or paper chains. When making the Advent "calendars" made of paper links, youngsters think of a good deed to write on each link. Then, as each day in December passes, they remove a link and perform that act of charity or forgiveness.

The Thomas family prefers to use the chocolate calendars and keeps one in each child's room. "Now when we use the Advent calendars, the kids understand that every day they open a door, that means we are one day closer to the celebration of Christmas," Thomas says. "I think it is a fun tradition they will always remember when they are older."