An Introduction to Calendars

Maya Calendar

Thirty days hath September,
April, June, and November.
All the rest have thirty-one
Excepting February alone—
Which hath but twenty-eight, in fine,
Till leap year gives it twenty-nine.

That convenient little rhyme above helps us remember the odd distribution of days in our various months. It also reminds us that February has but twenty-eight days in common years, while all of the other months have either thirty or thirty-one.
Why did the Western calendar’s architects short-change February by two or three days? A thirty-day February would provide a much more symmetrical means of marking the year’s progress. Two additional days beyond February’s twenty-eight could easily have been taken from a couple of thirty-one day months to give February its fair share.

Actually, having more thirty-day months in the year is hardly a new notion. Several thousand years ago, calendars of the earliest civilizations in Mesoamerica, Europe, Asia and Africa all had twelve thirty-day months in the year. Their calendar years each consisted of a nice, round, 360 days. Most of these calendars were developed independently of the others.
While this manner of modeling the year was followed in some parts of the world for many hundreds of years, it was abandoned in most places between the eighth and fourth centuries B.C. But old customs die hard. Some calendars based upon twelve thirty-day months are still in use today.
Why did most early peoples modify their tidy 360-day calendar? No doubt it was for practical reasons. Calendars have long been an indispensable part of everyday life—they tell us when to expect a change in seasons; they tell the farmer when to plow or plant; they tell the priest when to prepare for fasts or festivals; they provide a measure of years that have passed since some important event and they help us all plan for the future as well as keep track of passing time.
In the modern world, calendars have become indispensable for the financial community. Many debits and credits are scheduled for fixed dates within each month. Certificates of deposit have lengths that are various multiples of 30 days.