How long is a year? In other words, what is some basic history of calendar 
usage. When did we start using a 365 day year with occasional leap years? That 
kind of thing.

The use of a 365 day year with occassional leap years was introduced in 46 BC 
with the Julian calendar. The Julian Calendar was instigated by Julias Caesar 
and had commissioned the Alexandrian astronomer Sosigenes to revise the 
calendar system. Sosigenes used a tropical solar year, which calculates to 
365.25 days per year. This was slightly off, the actual tropical solar year is 
365.242199. In 1572 Pope Gregory III issued a "papal bull" to fix the problem 
that the Julian calendar was actually off by 10 full days. This discrepency is 
what caused there to be 10 days missing by the year 1572. The Jesuit astronomer 
Christopher Clavius undertook the Pope's decree and designed what is now known 
as the Gregorian calendar. It introduced the practice of having leap years for 
centennial years only if they were divisible by 400. So while 1700, 1800 and 
1900 were by the Julian calendar regarded as leap years, under the Gregorian 
calendar, they are not.
Paraphrased from webpage at http://www.ernie.cummings.net/calendar.htm#FOUR

Another webpage summarizes the introduction of the leap year in the Julian 
calendar:

"In order to put an end to the disorders arising from the negligence or 
ignorance of the pontiffs, [Julius] Caesar abolished the use of the lunar year 
and the intercalary month, and regulated the civil year entirely by the sun. 
With the advice and assistance of Sosigenes,[7] he fixed the mean length of the 
year at 365 1/4 days, and decreed that every fourth year should have 366 days, 
the other years having each 365."
http://es.rice.edu/ES/humsoc/Galileo/Things/gregorian_calendar.html

and for the Gregorian calendar:

"After the unification of the Papacy in Rome, in the fifteenth century, Popes 
began to consider calendar reform. After several false starts, a commission 
under the leadership of the Jesuit mathematician and astronomer Christoph 
Clavius (1537-1612) succeeded. Several technical changes were instituted having 
to do with the calculation of Easter, but the main change was simple. In 1582 
Pope Gregory XIII (hence the name Gregorian Calendar) ordered ten days to be 
dropped from October, thus restoring the vernalequinox at least to an average 
of the 20th of March, close to what it had been at the time of the Council of 
Nicea. In order to correct for the loss of one day every 130 years, the new 
calendar dropped three leap years every 400 years. Henceforth century years 
were leap years only if divisible by 400. 1600 and 2000 are leap years; 1700, 
1800 and 1900 are not."
http://es.rice.edu/ES/humsoc/Galileo/Things/gregorian_calendar.html


Additional calendar history websites:

History of the Western Calendar
http://ourworld.compuserve.com/homepages/khagen/CalHist.html

History of Astronomy: Calendars, Time and Chronology
http://www.astro.uni-bonn.de/~pbrosche/hist_astr/ha_items_calendar.html

A Variety of Information about Calendar History
http://www.ernie.cummings.net/calendar.htm

History of the Gregorian Calendar and Leap Year
http://www.mcs.drexel.edu/~gcmastra/mail/calendar.html

History of the Calendar
http://www.greenheart.com/billh/linked__.html


Google search terms used "calendar history greek", "calendar history"

Very interesting, there is a lot of information in those links! Thanks!