kalendars

A calendar is a system for arranging days into units such as years, months, and weeks, used to plan activities, schedule civil life, and mark festivals and historical anniversaries. The word calendar derives from Latin calendarium, meaning a book or register of accounts, and ultimately from calenda, the first day of the month in the old Roman market week.

There are several basic types: solar calendars, which count the solar year; lunar calendars, which count lunar months; and lunisolar calendars, which try to reconcile both cycles by adding days or months. The Gregorian calendar is a solar calendar; the Hebrew and Hindu calendars are lunisolar; the Islamic calendar is purely lunar.

A calendar's structure typically includes years, months, weeks, and days. Months vary in length from 28 to 31 days; weeks are a standard 7-day cycle in most cultures. Solar calendars approximate the tropical year by inserting leap days or leap months to keep seasons aligned.

Historically, calendars emerged in ancient Mesopotamia, Egypt, and the Indus Valley, with Roman and Greek systems contributing. The Julian calendar (45 BCE) introduced a 365-day year with a leap day every four years. The Gregorian reform (1582) refined leap year rules to better align with the tropical year, and adoption spread gradually around the world.

In modern civil life, the Gregorian calendar is the de facto international standard, and ISO 8601 defines a numeric date format (YYYY-MM-DD). Many cultures retain alternative calendars for religious or cultural purposes, such as the Hebrew, Islamic, and Hindu calendars.