Since the beginning, humans have tried to measure time in terms of days, weeks, and months, as a way to plan and predict events in their lives. There are most likely failed attempts to create calendars, but what you read most about are the calendars that survived through the centuries, evolving to present day calendars as well as making predictions about the future based on celestial alignments. It is believed by ancient astronaut theorists that the delineation of time into cycles - was helped by alien visitors who came here in the form of gods - in the ancient past - leaving behind signatures of their time here so humans would remember what was, what is, and what is coming - as one measures experience in linear time.

A calendar is a system of organizing days for a social, religious, commercial or administrative purpose. This organization is done by giving names to periods of time Ð typically days, weeks, months and years. The name given to each day is known as a date. Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycles of some astronomical phenomenon, such as the cycle of the sun, or the moon. Many civilizations and societies have devised a calendar, usually derived from other calendars on which they model their systems, suited to their particular needs.

A calendar is also a physical device (often paper). This is the most common usage of the word. Other similar types of calendars can include computerized systems, which can be set to remind the user of upcoming events and appointments. As a subset, calendar is also used to denote a list of particular set of planned events (for example, court calendar). The English word calendar is derived from the Latin word kalendae, which was the Latin name of the first day of every month.

There are various calendar systems - the simplest counts time periods from a reference date. This applies for the Julian day. Virtually the only possible variation is using a different reference date, in particular one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of addition and subtraction. Other calendars have one (or multiple) larger units of time.

There are solar calendars, lunar calendars and lunisolar calendars. An astronomical calendar is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the Second Temple. Such a calendar is also referred to as an observation-based calendar. The advantage of such a calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult.

An arithmetic calendar is one that is based on a strict set of rules; an example is the current Jewish calendar. Such a calendar is also referred to as a rule-based calendar. The advantage of such a calendar is the ease of calculating when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation. This limits the lifetime of an accurate arithmetic calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar.

Very commonly a calendar includes more than one type of cycle, or has both cyclic and acyclic elements. Many calendars incorporate simpler calendars as elements. For example, the rules of the Hebrew calendar depend on the seven-day week cycle (a very simple calendar), so the week is one of the cycles of the Hebrew calendar. It is also common to operate two calendars simultaneously, usually providing unrelated cycles, and the result may also be considered a more complex calendar. For example, the Gregorian calendar has no inherent dependence on the seven-day week, but in Western society the two are used together, and calendar tools indicate both the Gregorian date and the day of week.

The week cycle is shared by various calendar systems (although the significance of special days such as Friday, Saturday, and Sunday varies). Systems of leap days usually do not affect the week cycle. The week cycle was not even interrupted when 10, 11, 12, or 13 dates were skipped when the Julian calendar was replaced by the Gregorian calendar by various countries.

Calendars may be either complete or incomplete. Complete calendars provide a way of naming each consecutive day, while incomplete calendars do not. The early Roman calendar, which had no way of designating the days of the winter months other than to lump them together as "winter", is an example of an incomplete calendar, while the Gregorian calendar is an example of a complete calendar.

Calendars in widespread use today include the Gregorian calendar, which is the de facto international standard, and is used almost everywhere in the world for civil purposes, including in the People's Republic of China and India (along with the Indian national calendar). Due to the Gregorian calendar's obvious connotations of Western Christianity, non-Christians and even some Christians sometimes replace the traditional era notations "AD" and "BC" ("Anno Domini" and "Before Christ") with "CE" and "BCE" ("Common Era" and "Before Common Era").

The Islamic calendar or Muslim calendar or Hijri calendar is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in most of the Muslim countries (concurrently with the Gregorian calendar), and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals. The first year was the year during which the emigration of the Prophet Muhammad from Mecca to Medina, known as the Hijra, occurred. Each numbered year is designated either H for Hijra or AH for the Latin anno Hegirae (in the year of the Hijra). Being a purely lunar calendar, it is not synchronized with the seasons. With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately each 33 Islamic years.

The Hindu calendars are some of the most ancient calendars of the world. Eastern Christians of eastern Europe and western Asia used for a long time the Julian Calendar, that of the old Orthodox church, in countries like Russia. For over 1500 years, Westerners used the Julian Calendar as well.

While the Gregorian calendar is widely used in Israel's business and day-to-day affairs, the Hebrew calendar, used by Jews worldwide for religious and cultural affairs, also influences civil matters in Israel (such as national holidays) and can be used there for business dealings (such as for the dating of checks).

The Chinese, Hebrew, Hindu, and Julian calendars are widely used for religious and/or social purposes.

The Iranian (Persian) calendar is used in Iran and some parts of Afghanistan.

The Ethiopian calendar or Ethiopic calendar is the principal calendar used in Ethiopia and Eritrea.

In Thailand, where the Thai solar calendar is used, the months and days have adopted the western standard, although the years are still based on the traditional Buddhist calendar. Bahai's worldwide use the Baha'i calendar.

Even where there is a commonly used calendar such as the Gregorian calendar, alternate calendars may also be used, such as a fiscal calendar or the astronomical year numbering system.

A fiscal calendar (such as a 4/4/5 calendar) fixes each month at a specific number of weeks to facilitate comparisons from month to month and year to year. January always has exactly 4 weeks (Sunday through Saturday), February has 4 weeks, March has 5 weeks, etc. Note that this calendar will normally need to add a 53rd week to every 5th or 6th year, which might be added to December or might not be, depending on how the organization uses those dates. There exists an international standard way to do this (the ISO week). The ISO week starts on a Monday, and ends on a Sunday. Week 1 is always the week that contains 4 January in the Gregorian calendar. Fiscal calendars are also used by businesses. This is where the fiscal year is just any set of 12 months. This set of 12 months can start and end at any point on the Gregorian calendar. This is the most common usage of fiscal calendars. Read more

October 5-14 1582: The Ten Days That Didn't Happen   IFL Science - October 18, 2023
In 1582, Pope Gregory XIII introduced the Gregorian calendar. Prior to this, most of the Roman world and Europe had used the Julian calendar, introduced by Julius Caesar in 45 BCE.