If you are interested in the history, I found an online PDF of The Discoverers (www.dillgroup.ucsf.edu). Start reading at page 4 to learn all about how humans have come to mark time as we have.

One problem with the lunar calendar is that it gets out of sync with the seasons with each passing year. The ancient Egyptians first "escaped" the lunar calendar by marking the passage of time with the Nile; which conveniently lined up with a 365 day year... almost.

Since the solar year, of course, is not precisely 365 days, the Egyptian year of 365 days would, over the centuries, become a "wandering year" with each named month gradually occurring in a different season. The discrepancy was so small that it took many years, far longer than any one person's lifetime, for the error to disturb daily life. Each month moved through all the seasons in fourteen hundred and sixty years. Still, this Egyptian calendar served so much better than any other known at the time that it was adopted by Julius Caesar to make his Julian calendar. It survived the Middle Ages and was still used by Copernicus in his planetary tables in the sixteenth century.

The reform of the calendar by Pope Gregory XIII was needed because the year that Julius Caesar had borrowed from the Egyptians, and which had ruled Western civilization since then, was not a precise enough measure of the solar cycle. The actual solar year—the time required for the earth to complete an orbit around the sun—is 365 days, 5 hours, 48 minutes, and 46 seconds. This was some 11 minutes and 14 seconds less than the 365 V\ days in the Egyptian year. As a result, dates on the calendar gradually lost their intended relation to solar events and to the seasons. The crucial date, the vernal equinox, from which Easter was calculated, had been fixed by the First Council of Nicaea at March 21. But the accumulating inaccuracy of the Julian calendar meant that by 1582 the vernal equinox was actually occurring on March 11. Pope Gregory XIII, though notorious now for his public Thanksgiving for the brutal massacre of Protestants in Paris on Saint Bartholomew's Day (1572), was in some matters an energetic reformer. He determined to set the calendar straight. Climaxing a movement for calendar reform which had been developing for at least a century, in 1582 Pope Gregory ordained that October 4 was to be followed by October 15. This meant, too, that in the next year the vernal equinox would occur, as the solar calendar of seasons required, on March 21. In this way the seasonal year was restored to what it had been in 325. The leap years of the old Julian calendar were readjusted. To prevent the accumulation of another n-minute-a-year discrepancy, the Gregorian calendar omitted the leap day from years ending in hundreds, unless they were divisible by 400. This produced the modern calendar by which the West still lives.