Surely, over the generations and making primitive consciousness of its existence, began to observe and measure the lunar cycles. Lunations were thus emerging as the current uneven months. A practical way for that primitive man, to have sets of days.
But there was another cycle in nature that caused the need to understand, most likely, by a look of pure survival. The weather ranged from periods of intense heat to others of virulent cold. The sun warmed not always equal or always loomed in the sky up so high, the shadows varied from a few moon cycles, day and night did not last long ... They looked like the stars of the night and saw that some had appeared on the horizon at some times and at other stations were hiding underground. This sensed the annual cycle and with the development and support of mathematical knowledge, they discovered four milestones that shaped the year: two solstices and two equinoxes. Among them, many civilizations defined the four seasons.
But the three cycles discovered, that of Earth
To finish simplify the subject, the as mathematics and astronomy evolved, detect subtle deviations of the cycles. Then, about putting things fit a thirteenth month from time to time, others two days added to each lunation to better fit the 12-lunar cycles in a solar cycle, others jumped a handful of days back in the sixteenth century to "fit "Easter
Indeed, the calendar is a human invention that has been used to concatenate the facts of your story. So today has nothing special than being the first day of the year in the Gregorian calendar which, coincidentally, around here we count the days of our history. However, aware of the farce that society has imposed on us, we will use the holiday to celebrate turning stay here on Earth