Each calendar is created from an A4 sheet folded so that it has three faces, each approximately 90mm high, plus a 'tab' of around 30mm. A piece of card is glued to the back of one of the faces so that, when assembled, the tab is inserted between the calendar face and the card. This inner card carries assembly information and can easily have a personal greeting printed on it. See the example here:

Note, you can't use the Enter key to start a new line - instead you should insert a vertical line to mark the end of a line. The key should be next to the letter z on the keyboard.