Calendarios

Calendarios, or calendars, are systems for organizing days into units of time—days, weeks, months, and years—used to plan events, track time, and regulate social and religious life. They can be civil, for everyday administration, or religious, to observe holy days. Most calendars are linked to the cycles of the sun or the Moon and may be solar, lunar, or lunisolar. They often include leap years or intercalary periods to keep the calendar aligned with the seasons.

Historically, calendars emerged from agricultural needs and astronomical observations. The Julian calendar, introduced by Julius Caesar in 45 BCE, established a 365-day year with a 1‑day leap year every four years. By the 16th century a drift of about 10 days prompted the Gregorian reform of 1582, which dropped 10 days and refined leap-year rules: years divisible by 4 are leap years, except that centennial years are not unless divisible by 400. The reform spread gradually and became the basis for the modern civil calendar used in most of the world.

Major calendar types include solar calendars, such as the Gregorian calendar, which aims to approximate the solar year; lunar calendars, such as the Islamic calendar, which follows the cycles of the Moon; and lunisolar calendars, such as the Hebrew and traditional Chinese calendars, which combine both rhythms and may insert leap months or days to stay aligned with the seasons.

In contemporary use, calendars exist in physical form and as digital tools, integrated with scheduling software and time-zone data. They support cultural and religious observances and are standardized for interchange in software, printing, and everyday life.