The ancient Romans called the first day of the month "kalenda". Hence the word "calendar" came from as a way of dividing the year into time intervals with a convenient frequency.
The calendar allows you to fix dates and measure time intervals. This is necessary to register events in chronological order. For a long time, calendars have been used to designate church holidays - including those that do not have an exact date, as in the case of Easter. In secular life, wages, interest payments and other obligations are also tied to time intervals.
The main types of calendars are solar, lunar and lunisolar. The length of the day is determined by the rotation of the Earth around its axis. The lunar month is tied to the rotation of the moon around the earth. The solar year is given by the rotation of the Earth around the Sun.
The ancient Egyptians, Mayans and most modern countries adhere to the solar calendar. It is tied to the length of the solar year, in which 365, 2422 days. The civil calendar has been adjusted for the integer 365, and the missing fractional part is taken into account by adding one day to the leap year.
Muslims use the lunar calendar, in which the length of the year is 354 days. This is 11 days shorter than the solar year, and causes inconvenience in public life.
In the lunisolar calendar, an attempt is made to match the length of the solar year with the lunar months by means of adjustments. This is the Jewish calendar officially adopted in Israel.
In different historical periods, attempts have been made to improve the timing. The problem is that both the solar year and the lunar month contain fractional parts that can be counted differently. This is done using corrections at regular intervals.
Greek calendar. The year consisted of 354 days. Every 8 years, 90 days were added to it, divided by three months.
The Roman calendar consisted of 10 months, then two more were added. Around 451 BC. the beginning of the year was postponed to January 1 and the sequence of months led to the present form.
Julian calendar. At first, the dates did not coincide with the natural seasons. After the reform of Julius Caesar, a leap year appeared. The Julian calendar is called the "old style".
Augustian calendar. When Caesar died, the leap month was added not every four years, but every three. This mistake was corrected by Emperor Augustus. He also changed the duration of some months. As a result, the now familiar system appeared.
Chinese calendar. Several millennia BC. Emperor Yao ordered the creation of a calendar suitable for agricultural work. Until 1930, the peasants used the ancient calendar, then it was banned.
Gregorian calendar. Pope Gregory XIII supplemented the Julian calendar, and March 21 became the day of the vernal equinox. Since 1582, the so-called new style appeared. The correction of the dates caused confusion because Gregory XIII ordered the correction of the previous dates. Now the Gregorian calendar is used in Russia, the USA and other countries. The Gregorian calendar is consistent with natural phenomena, but it also has drawbacks. There is talk of improving and reforming the calendar.
The world calendar was developed in 1914. In it, the week and year always start on Sunday.
Edwards' perpetual calendar is divided into quarters. Each week starts on Monday, which is convenient for business. Friday does not fall on the 13th. In the United States, they even submitted a bill to the House of Representatives to switch to this calendar.