Here's a series of pages devoted to different calendars: http://webexhibits.org/calendars/calendar-mayan.html
Another inventory of calendars: http://www.projectpluto.com/calendar.htm
About Julius Caesar and leap days http://antwrp.gsfc.nasa.gov/apod/ap960229.html
“Tibaldo and the Hole in the Calendar" by Abner Shimony, 165 pp, Copernicus 1998. The book tells the story of a boy in 16th-century Italy whose birthday celebration was set for one of the “lost" days, skipped over by the one-time jump in the calendar which Pope Gregory the 13th ordered. Reviewed by Stephen Battersby in Nature, p. 460, 3 April 1998, and by David Mermin in Physics Today, p. 63, June 1998.
Wikipedia has an extensive page on the Maya Calendar: en.Wikipedia.org/wiki/Maya_calendar
To learn more about the Maya and Venus in informal writing, see the chapter “Bringing Culture to the Physicists,” p. 313 in “Surely You're Joking, Mr. Feynman!" by the great physicist Richard Feynman.