Perpetual calendar

A perpetual calendar is a tool designed to display the correct day of the week for any date over many years without modification. It accounts for leap years and the variations in the calendar over time, making it useful for planning or historical reference. Typically, it consists of a set of rotating or sliding parts that align to show the day corresponding to a given date, or a chart that adjusts for leap year cycles. Essentially, it automates the manual calculation of weekdays for future or past dates, saving time and reducing errors across long periods.