Calendar

A calendar is a system used to organize and name periods of time, such as days, weeks, months, and years. It helps people plan activities, remember events, and coordinate dates. Most countries today use the Gregorian calendar for everyday purposes. It has 12 months and normally contains 365 days in a year.

Every fourth year is usually a leap year with 366 days, although years divisible by 100 are not leap years unless they are also divisible by 400. This adjustment helps keep the calendar aligned with Earth's revolution around the Sun, which takes about 365.24 days. A calendar year is therefore not exactly the same length as the time taken by Earth to complete one orbit.

Calendars have existed in many forms throughout history, and different cultures have developed systems based on the movements of the Sun, the Moon, or both. A solar calendar follows the cycle of Earth's movement around the Sun, while a lunar calendar is based mainly on the phases of the Moon. Some calendars combine both approaches and are called lunisolar calendars.

In the Gregorian system, weeks contain seven days, but the number of days in a month varies between 28 and 31. Calendars can also mark seasons, holidays, religious celebrations, school periods, or important historical dates. Beyond being a simple table of dates, a calendar is a practical way for societies to organize time and share a common reference for when events take place.