Perpetual calendar

An early example of a perpetual calendar for practical use is found in the Nürnberger Handschrift GNM 3227a.

This is the first known instance of a tabular form of perpetual calendar allowing the calculation of the moveable feasts that became popular during the 15th century.

[5] The chapel Cappella dei Mercanti, Turin contains a perpetual calendar machine made by Giovanni Plana using rotating drums.

Their purpose, in part, is to eliminate the need for perpetual calendar tables, algorithms, and computation devices.

Even though the individual operations in the formulas can be very efficiently implemented in software, they are too complicated for most people to perform all of the arithmetic mentally.

Zeller's congruence, a well-known algorithm for finding the day of the week for any date, explicitly defines January and February as the "13th" and "14th" months of the previous year to take advantage of this regularity, but the month-dependent calculation is still very complicated for mental arithmetic: Instead, a table-based perpetual calendar provides a simple lookup mechanism to find offset for the day of the week for the first day of each month.

To simplify the table, in a leap year January and February must either be treated as a separate year or have extra entries in the month table: The following calendar works for any date from 15 October 1582 onwards, but only for Gregorian calendar dates.

Gregorian 31 March 2006: Greg century 20(c) and year 06(y) meet at A in the table of Latin square.