Obviously, there are seven days in a week.
Realistically, it's an arbitrary grouping of days. And there is a grouping of these weeks into a month. And a grouping of these months into a year. To become even more granular...what's a day. Obviously of all these divisions, day is the easiest to understand. However, do you measure the start of a day by the opposite of the highest point of the sun (like we kind of do) or does it start with the sunrise or the sunset? And then what about divisions within days? Twenty four hours containing 60 minutes which each contain 60 seconds is all very arbitrary.
For those of you writing anything relating to our world then this doesn't really matter, but for those other fantasy writers out there, perhaps building a calendar will add a bit more believability to your world. That being said, you don't want huge nerdy chunks of exposition explaining your calendar or your religion, but having a solid handle on how this works and just showing your reader pieces of it as needed can allude to a complex and vast society that they are only able to see a sliver of and this will leave them wanting more.
Last year I wrote an article (as part of a group project although I did most of the research and all the writing). The following are excerpts. If you find this interesting and want more information (the full paper details nearly a dozen different cultures and their calendars) feel free to email me to request the full text.
The concept of time--be it through hours, days, weeks, months or years--is a fundamental aspect of human life. We need to know when to plant and when to harvest. We need to know how much food to gather in order to survive the long winter. We need to plan when to meet up with other groups or peoples in our culture and we need to measure the lengths of our lives. These needs can be solved by use of a calendar. A calendar of various levels of complexity existed in every culture. While each culture began with the same clues--moon, stars and sun--different cultures observed different celestial and earthly occurrences depending upon their location on the globe, their culture and their history. But, when trying to map the occurrences in the cosmos to an earthly calendar, problems arise because celestial events do not occur in perfect ratios of each other. A lunar month contains 29.53 days. A solar year contains 365.2507 days. A lunar year of twelve months contains 354.367 days, which is around 11 days short of a solar year, causing the months to slowly pull away from the seasons.
Thus, compromises are needed in order to create a usable calendar. Different compromises seemed reasonable to different societies depending on their mathematical sophistication, their understanding of astronomy, and their culture. A wide variety of calendar systems have been developed throughout history, each with its own set of trade-offs that reflect how each culture understood math and the cosmos. Some cultures created a calendar based upon the moon, but annual events such as the solstice fell on different days each year. Other calendars respond this by basing themselves upon the sun, resulting in an arbitrary or unrelated lunar cycle. Others interject extra months every few years, a technique known as intercalation, to keep the seasons occurring at approximately the same time each year. Still others, such as our modern calendar, follow the solar calendar by adding days and even seconds to keep the calendar aligned.
But, all of the first calendars were based upon the moon which has been heralded as the "first chronometer" (Rice 38). The moon, in addition to waxing and waning in a predictable, very obvious and relatively short pattern, also affects the tides. In coastal areas the correlation between new and full moons and a very high tide contributed to civilizations' central view of the moon for calendar purposes and caused them to associate the moon with their deities. As time went on groups had to either live with a calendar that did not predict yearly occurrences such as seasons or change the calendar to accommodate the year.
The calendar often will also contribute to holidays. Most cultures, for example, celebrate their new year.
So, if this has gotten you thinking a little bit just drop me an email at lauren.amundson (at) gmail (dot) com and I can share the full text of the paper with you. I've got details from why the Egyptians first started understanding the solar year (Nile floods yearly at the same time and colloates with the rising of the star Sirus) to why the Mayans didn't think the world would end in 2012.
Every culture has a calendar. What's yours? Do any of you fantasy writers out there have a different calendar or any special holidays? My culture, for example, celebrates the summer solstice as their most holy day.