The calendar is a conventional system of time calculation and division on a yearly basis. It originates from astronomical events to count and group in periodic intervals (months and weeks) the minimum time unit of a calendar, the day.
Starting from the days, the calendar is organized in a week of seven days; in a month, which duration is at least but mainly more than four weeks; in the year, made of twelve months. The result is therefore a list of dates and names for the various periods during the year.
The word “calendar” comes from the Latin word "calendae", the first day of each month, which in turn derives from the Greek καλω, to call, proclaim. It means in Latin "to summon, to proclaim", and evokes the call, because at Kalends or Calendae days the Comitia Calata were held, where in front of the summoned people, it was proclaimed the first day of the month and all the days dedicated to feasts, rituals and laws during the current month.
In the Roman calendar, the days are counted like a countdown, ordering the days preceding special dates, which mark every month. Hence the creation of the calendar as an ordered list, the calendarium.
The size of the calendar is the year and the term comes from the ancient old German “gear/jer” meaning year, season, period of time . The related Greek word ὥρα, means season and specifically "harvest", "plenty, prosperity". The Latin word for year is annus meaning around, as a circle and not by chance the word is used in the word annulus, ring.
The astronomical observations of the motion of the Sun and the Moon are the basis for all the measured period of time:
• the year derives from the period of the Earth’s revolution in its orbit around the Sun;
• the month derives from the lunar cycle comprehensive of all the moon phases, by which the moon complete a cycle and turns to the same phase. The civil month can be of 28 up to 31 days;
• the week derives from a moon phase duration, four of which make up the month. Actually the average length of a lunar phase is approximately 7.3826 days, hence the phases may begin with the time on different days of the week;
• the day derives from the period of the Earth's rotation around its axis;
The month currently in use is actually mostly of 30 or 31 days, longer than the common 28 days of four weeks and longer then the whole lunar cycle of 29 days and half, as given by the duration of the lunar phase, more than seven days, as said.
Lunar, lunisolar and solar calendars
The calendars can be classified in the following types:
It is the oldest type and is based on the length of the lunar month, about 29 and half days. The month begins with the new moon. The lunar month, being shorter than the calendar month (approximately 30-31 days), causes in a year a lag of about 11 days, so that the seasons begin earlier from year to year. Main example is the Islamic calendar.
It is an evolution of the previous and relies on both the duration of the solar year and the lunar month. The difference between them is aligned alternating periodically one year of 13 months to 12 months years. The start date of the seasons moves from year to year, but it is restricted within12-13 days from a fixed date. Actually a year of 365 days requires 12.3683 lunar months. The alignment is restored every Meton cycle, which states that 19 solar years correspond to 235 lunar months. Main examples are the Hebrew, Chinese, ancient Greeks calendars and the Celtic calendar of Coligny.
It is the most complex but also the more accurate calendar than the previous ones. It is made up of 365 days as the civil year, even if it is originated from the solar or tropical year (see link), the period of the Earth’s motion in its orbit around the Sun, of exactly 365.2 days. Because of this approximation, it is necessary to add one day every four years. This is the leap year of 366 days, bringing February to 29 days. The benefit is to have is a far more stable time computation with less variation and the succession of seasons at more or less the same day. (See link) Main examples are the Julian and the Gregorian calendars, the last one is the most used in the world.