kalendern

Kalendern is the system used to organize days into years, months, and weeks for social, religious, economic, and administrative purposes. Calendars can be solar, lunar, or lunisolar, reflecting different astronomical cycles and cultural practices. A solar calendar aims to keep years aligned with the Earth's orbit around the Sun, while a lunar calendar tracks the phases of the Moon, and a lunisolar calendar combines both, adding adjustments to keep months aligned with the seasons.

The most widely used calendar today is the Gregorian calendar, a solar calendar introduced in 1582 to correct drift in the earlier Julian calendar. It uses 12 months with 28 to 31 days and adds a leap day in most years divisible by four, with century years skipped unless divisible by 400. This system provides a year of about 365.2425 days on average.

Lunisolar calendars, such as the Hebrew and traditional Chinese calendars, keep lunar months but insert leap months periodically to stay synchronized with the solar year, so that holidays fall in the same seasons. The Islamic calendar is strictly lunar, consisting of 354 or 355 days in a year, and its months drift relative to the seasons because it does not add leap months.

Other notable calendars include the Persian (Iranian) solar calendar, Hindu lunisolar calendars, and the Ethiopian calendar, each with unique eras, month names, and cycle rules. Modern usage often includes ISO date representations and week numbering for international coordination. Calendars underpin scheduling, timekeeping, education, and cultural rituals worldwide.