A calendar comprising two sheets with one laid flat against the other and the sheets being moveable one over the other. One sheet bears the numbers "1" to "31" marked on it and the other sheet bears markings such that the numbers can be brought into registration with respective ones of the markings, successive markings being indicative of successive days of the week. Each month is represented by an indication on the sheet bearing the said markings adjacent to a marking indicative of that day of the week on which the first day of that month falls. Apertures are formed in the intended upper one of the sheets and further marks are provided on the intended underneath sheet such that when the indication of a given month on one of the sheets is brought adjacent to the number one on the other sheet, a mark appears through one of the apertures which is in registration with that number on the said one of the sheets which corresponds with the last day of the given month.