The calendar changes every year primarily due to the fact that Earth takes about 365.25 days to complete one full orbit around the Sun. To account for this extra quarter of a day, we add an extra day, known as a leap day, to our calendar every four years, creating a leap year with 366 days.
For many cultures, calendars are essential for organizing time, determining the dates of important events, seasons, and festivals. The Gregorian calendar, which is the most widely used today, was introduced in 1582 and refined the system of leap years to ensure it remained accurate over long periods.
However, the calendar more commonly ‘changes’ each year visually because each month starts on a different day of the week, making the layout of calendar pages look different from year to year. This shift is based on the sequence of weekdays and how they align with the number of days in each month.
In summary, the calendar changes every year due to the nature of Earth’s orbit, adjustments for accuracy, and the varying way months align with the weekly cycle.