Abstract: In the field of calendars the rule is a unit representation of the year from the 1 January—the first day of the year—till the 31 December—the last day—(an objective representation of time which follows the Gregorian calendar); the Anniversary Calendar, on the contrary, represents the annual period (of 365 days or, in case of leap years, of 366 days, considering that the solar year lasts 365 days, 5 hours and 49 minutes) starting from a fixed day and month (of the Gregorian calendar) in which an event is celebrated, until the eve of the annual anniversary day of the same event. The annual period so represented is divided up into 12 rectangles (each one containing 31 boxes). The first day of each rectangle will have the same number of the day of the event, except few exceptions indicated in the detailed description.