The first calendars in human history were intended to determine the cycles of the sun and moons to know the seasons and time. From the ancient Egyptians, Sumerians, and Babylonians we have the first calendars that were based on the observation of the natural world around them to know which are the best seasons to plan the periods of agriculture and the corresponding harvest, that is, to be able to know if the weather conditions they will be suitable in the corresponding weather.

Technology evolved, man’s curiosity to know the world and tell time grew, and much more precise measurements were made so that modern calendars could synchronize the Earth’s orbit around our star, the Sun.

The Roman calendar, specifically the Julian calendar, from Julius Caesar about 2070 years before the present era, reformed the calendars of the time by adding one more day every four years, thus creating leap years.

A leap year is a year that has 366 days instead of 365, giving the extra day in the month of February, which is called February 29.

But why does this happen? The natural rotation of the Earth around the sun is not exactly one year (365 days) but it is one year and a quarter of a day, i.e. 365.25 days. Every four years these quarters of the day are added up, and the extra day was determined to bring the length of the human calendar into line with the natural time of the Earth’s rotation.

What would have happened if this had not been done?
After a few decades, the seasons as we define them would shift and agricultural production would be affected as well as social events in the human calendar (Christmas, holidays) would be disrupted and we would lose time in relation to the Earth’s natural cycles of sunrise and sunset. west.

Before the addition of the extra day, they were trying to correct the calendar, readjusting the months according to the cycle of the moon or making additional corrections, adding months, and so on, but Caesar with his arrangement, made the measurement of time more accurate than with the seasons and the constellations.
https://en.wikipedia.org/wiki/1700

Neolithic building that aligns with the spring and autumn equinoxes, when the sun passes over its stone “pyramids”. Perhaps from the first constructions that measured time, perhaps from the first astronomical observatories. Credits: Rupert Jones/Flickr

The Aztec solar calendar, a year had 260 days and was synchronized with various astronomical events and religious festivals. It was divided into smaller sections (something like months) each of which had 20 days. Each day of had its name, symbol, and meaning, while at the same time, there was another division for the sections that each had their properties. When 20 days over 13 divisions had passed, a year had been completed. Credits: Dennis Jarvis, CC

https://www.worldhistory.org/article/896/the-aztec-calendar/#:~:text=The%20Aztecs%20used%20a%20sacred,originally%20based%20on%20astronomical%20observations.

The first calendars followed the phases of the moon to divide the year, having twelve months of 29 or 30 days.