Many Advent calendars and large cards shaped like a rectangle, with little "windows" that can be opened. There are usually 24 windows on a card, one for each day from December 1 to the day before Christmas. Each window has a number, and you open one each day. For example, on December 16, you would open the window with a '16' on it. There are sometimes pictures, poems, or even chocolate behind each window. Other calendars have Bible verses or a prayer behind the windows, which Christians use to celebrate their Advent as part of their religion. Advent calendars can also have puzzles or games behind the windows.