Making a calendar
I’m in the early stages of setting up a sandbox setting for potentially multiple groups with variable player rosters. In brainstorming how I would organise my records for such a persistent setting independent of the PC parties, I realised I needed to create a calendar. I’d need to keep track of when things happened so that I could restock dungeons believably, track the progress of event lines the PCs neglect to interrupt, determine the likelihood of items on fallen PCs remaining where they died, and sundry little play logbook things I like keeping track of like local weather.
The last time I ran a game where bothering to track time made a difference I already had a calendar provided by the Forgotten Realms setting; the time previous was more than a decade back and I fudged with an expired real calendar for some year in the 1990s.
But I like building settings and the bits that make them unique, so I didn’t want to use a real calendar, and obviously I wasn’t going to be handed a published fantasy calendar.
I’m also lazy, and I wanted a calendar that was conceptually easy while not being dull. So here’s how I went about building one.
I had a few requirements for my calendar that were dictated purely by usability and personal convenience. Sure, I could make something arbitrarily complex, but I wanted something that would at least have a chance of being comprehensible to players who will frequently have more important things on their mind than what month it is, let alone whether it was a month of short days or how many feast days there were coming up.
I wanted to have regular months. No variable days per month, no leap days, no funkiness from one month to the next.
I wanted to have a year roughly equal to 365 days. I’ve read novels where the year was a very different length from ours, and it takes mental gymnastics to follow along whenever time is relevant to the plot. I wanted “a year” to mean a year to my players.
Ditto weeks. If possible, I wanted 7-day weeks so that when an NPC says, “Your sword will be ready in a week, m’lady,” my players would immediately know what that meant without having to ask me (again and again) how long a week is in my weird funky calendar.
Months I wanted to be in the rough neighbourhood of real-world month, but I wasn’t going to push this one too hard because…
I didn’t want too many months. Players are only human, and humans deal best with quantities that are roughly seven in number, give or take. I didn’t want to have 12 months, really, as that’s just too many to ask players to care about paying attention to. My desire for the calendar to be meaningful in play to others than myself would best be served by having the fewest number of months so that players actually remembered them and had a rough idea of what they meant without having to constantly say, “Marpenoth is roughly like March”. 
I also had some stylistic preferences that I wanted to fit into the calendar.
There would be the usual four seasons. Seasons are usually more relevant to play because villages and weather react to the turn of seasons, not arbitrary month divisions.
Months should relate directly to the seasons. A system that is useful and comprehensible to farmers rather than an hurdle to properly timing plantings and harvest seems much more likely to be in widespread use. If I want to have the date come up naturally in-game in a useful way for the PCs, having NPCs using the calendar in their daily lives will better convey date information than me shoving it at them as meta-game information.
I like how the Forgotten Realms’ calendar includes days that are not part of any month or week, and these are culturally meaningful days. This sort of thing can also provide the wiggle room necessary to make some of the usability specs happen.
I wanted the division between years to be the last day of winter and the first day of spring, rather than the astronomical winter solstice. 
So I built a calendar with 7-day weeks, six weeks to a month, one extra day-of-rest “feast day” per two weeks, 360 days to the year, two months to a season, and eight months to a year.
It’s not terribly exciting, but I wasn’t going for exciting. It accomplishes the primary goal of giving me something easily-understandable to write in the headings of each day’s entry in an adventure log.
As a bonus it has all these interesting “non-calendar” days for me to play with. Some of those days are going to be special: the one between Early Summer and Late Summer is obviously Midsummer’s Day; the last day of Winter (well, not really part of the Winter months nor part of the Spring months) is some kind of year-death or new year’s day. Similarly, there are obvious prospects for harvest festivals and holy days. Those non-calendar days that aren’t claimed by religions or seasonal celebrations are going to be plain old feast days or market days or whatever sort of day of rest is culturally appropriate to the setting.
You’ll notice the names of the months aren’t marked. There’s an early and a late month in each season, and to avoid the Marpenoth Problem I’m going to assume that the inhabitants of this world are pragmatic and never saw a need to name the months with anything especial or non-obvious. So we have Late Fall and Early Summer, or New Spring and Old Winter, as references for month names. Really, any fantasy-sounding names I created would, if I were being sensible about it, mean those terms anyway, so made-up names would just be imposing a barrier between the players and their ability to reference time.
If the inhabitants are that pragmatic, and since I’m making it all up anyway and might as well have the world be gameable where it’s not implausible, I figure the days of the month are just going to be tracked with numbers. For spice, I might have some people count “the 10th of Early Summer” and other people count “the 3rd of two of Early Summer”. I kind of like the sound of the latter, though, so I think I’ll count time by default with “the [day]th of [week number] of [month]”. That way, “the 8th” will always be the strange day, the feast day, the day of celebration. I can use the other way of counting days as a marker for a particular culture or nation being odd and foreign, with their 32nds of Old Summer and Twelfths of New Winter. I figure they’ll just count the feast days in there too, as the 15th, 30th, and 45th.
For the reasons above I find this an eminently usable calendar for game records-keeping and for informing players, and has just enough flavour to say “fantasy” to me. It’s also pretty generic, being different from our own calendar while still matching our sense of year and week lengths. For a GM who needs a calendar this could be dropped into most implied settings with little to no work. 
If you want to use this in your home campaign you of course need no permission from me, but since I hate the headaches that licenses can bring I’ll just make it simple and put the calendar into the public domain. (If you use it I’d love to hear about it, but don’t count that an obligation!)
Marpenoth is actually equivalent to October in the Realms’ calendar which just goes to show that needing to keep the calendar mentally straight is an additional burden on anyone, GM included, and so likely to get neglected as a PITA.
Yes, I know that the visible demarcation between winter and spring is fuzzy and varies by year, which is why astronomical divisions were originally used in real-world calendars, but I’m planning on having a cosmology that isn’t based on planets orbiting a star in space. Without the inclination of a planet’s axis the solstice wouldn’t be determinable from the stars and would need good clocks instead. The winter/spring changeover is going to be conventional, fundamental to the establishment of the calendar, and keeping track of its return is the original function of the first calendar of this sort. As a convenience to me and time-tracking (because I don’t really want to figure out sunrise and sunset variations across the year but I still want sunrise, sunset, and midday to be practically meaningful), I may even make day/night lengths invariate between seasons, eliminating the concepts of solstice and equinox entirely.
Maybe shift the year-split to between the two winter months if that’s your taste, and the feast-days can easily by eliminated if you don’t like them by making the months a straightforward six weeks of seven days each for 42 days a month. And if you really like 12-month calendars you can easily redistribute the last two weeks and the first two weeks of each season’s months to make a third seasonal month, each of four weeks or 30 days (with feast days; 28 without).