When I was a child myself we always had an Advent calendar during December, and my siblings and I would take turns opening the windows as our excitement mounted with the approach of Christmas. Now I have children of my own I like to keep this tradition going, and this calendar is perfect for this purpose. The windows open easily and have cute little Christmas themed pictures, like gingerbread men, decorations and so on.
My tip for hanging this calendar is to use a strip of clear tape along the top edge only. This enables the calendar to be partially flipped up when opening the windows, as sometimes a little pressure on the back of the window makes it easier to open.
Note that this is not a religious themed calendar, as you may guess from the Santa picture.