In too many campaign settings, calendars are either ignored (leaving the game master to come up with something on his own and hope that the setting's designer doesn't come up with something completely at odds later on) or are too "easy". Months are all exactly the same number of days, which happen to be divided into full weeks, and the sun and moon(s) obey the clockwork precision needed for the game master's calendar to be perfectly symmetrical, orderly, and reusable in all years.
Historically, calendars are very messy things. The orbits of heavenly bodies rarely acquiesce to humanity's love of even numbers and symmetry. Not every civilization had weeks of seven days (or even weeks at all), and many were constantly tinkering with their calendars to make them more accurate, less complex, more in line with nature or what religious teachings taught about nature, or even for use as political tools. Even the most basic things, such as when the year actually begins, are subject to change.
Even the modern Gregorian calendar wasn't adopted until 1752. To this day it's not universally accepted, and some still use the Julian calendar or some other calendar altogether (Chinese, Muslims, Hindus, Jews, etc.).
So for my campaign I wanted something messy. Something that had a little character. So I came up with this (click to embiggen):