Us human beings like marking the passage of time and the repetition of time-based cycles. Mostly this is because our world moves in such cycles: we have sun-earth days, moon-earth months and earth-sun years. And most of the time, for your characters in your writing, these will also exist. But sometimes our Gregorian Calendar doesn’t work in a setting that isn’t Earth as we know it. After all, in a world where there has never been a Pope nor a Catholic Church nor a Roman Empire, why should their calender look exactly like ours? And why would it?
The solution is to roll your own. This is not nearly as daunting as it sounds. Many stories don’t need the reader to know an accurate timeline in days of what happens when. Even in David Eddings‘ The Belgariad, one of the classics of modern adventure fantasy, we don’t hear about months: we hear about seasons. For a society (and main characters) that is mostly tightly wedded to the farming cycle, this makes sense.
It turns out that even in our own world’s history, people have been marking off parts of the year by the seasons longer than they have been using months. In fact, there are four days of the year that perfect for this: the two solstices and the two equinoxes. And because they are tied to the movement of the earth around the sun, they always align with the seasons. For centuries, the Winter Solstice was celebrated as the turning point of Winter (which it is). It heralds the end of the preparation for the rest of winter with stockpiles of food and supplies. Likewise, the Spring Equinox, Summer Solstice and Autumn Equinox were equally important times during the year.
But sometimes a period of 91 days is too long to organise your life around. Early man would have noticed our nearest celestial body and found marking the days by the moon cycles was sometimes worthwhile, too. The very concept of “month” comes from “a moons’ worth of time”.
However, the moon’s cycle doesn’t fit into the sun’s cycle. So you have two conflicting ways of marking the passage of days – and man usually wants to use both. So a compromise has to be made. That’s where we get our modern months – shoe-horned into the year. It is notable that the Jewish calendar divides its year into months differently so that they more closely track the moon.
But our months are a political solution. In a fantasy world, the politics will be different. If you need months in your story, you could invent your own.
Tolkien had fun with his Hobbit calendar. It wasn’t really needed for the story, which is why the description of it is hiding in an appendix, but it provides a cute piece of additional characterisation for how Hobbits think. It is based on our own calendar, but shifted slightly so that Mid-Summer lands between two months instead of in the middle. They are also renamed to suit the way the seasons change. Then he solves the leap-year problem by putting extra days outside the months, and then removes them from the week, too, so that the week days don’t shift around from year to year. The Hobbits liked tidying that up.
Ah … I mentioned weeks. Weeks are a way of defining an even shorter period of time than a month. And are another division that don’t match either years or months. Yet we continue to use them. They are useful when a regular activity needs to be less often than daily but more often than once a month, such as religious services or village market days. But that’s no reason for your fantasy world to use weeks, or for them to even be seven days. Certain parts of tribal Africa have weeks of four days, for instance, and the French had ten-day weeks for a time during the French Revolution.
Whether you keep weeks, months and years or modify them slightly or a lot, one thing that must be borne in mind is that unless the politics of your calendar itself is the story, remember that this is essentially background information. It falls into the same as the detailed history of medicinal plants. It is colour to the descriptions. Rarely anything more.