Historical early calendars were mostly based on star positions and farming schedules (I believe early calendars only had 10 months because counting days just didn’t matter during the rest of the year). That being said, notable dates for a tabletop campaign are better put “on the run” (like, today just happens to be a holiday and all shops are closed so hey you have to spend the night in the inn AND IS IT HAUNTED) than recorded as one more thing to keep track of.

The AD&D DMG has a section on this topic, pointing out that seasons affect adventuring (weather conditions and temperature), as do moon phases (at the very least lycanthropes are traditionally affected by the phases of the moon. The Dragonlance campaign setting took the phases of the moons into account in a unique way, as the moons of Krynn have a direct effect on the magic users of that world. Krynn’s days and months were even given different names across different cultures and regions in one of the campaign guides–though I have no direct link to this information to share. Pathfinder’s Golarion setting has a calendar as well. Forgotten Realms has what appears to be a fairly detailed calendar. Rokugan, setting of Legend of the Five Rings and the 3rd Edition Oriental Adventures, has been given a calendar as well. Eberron was also given the calendar treatment. In fact, a google search for just about any published campaign setting’s calendar turns up results–and only occasionally are they results created by fans alone.

Quite aside from that, in a sandbox-style campaign a calendar can be useful in providing options and impetus for players in making decisions on what to do and where to go at a given time. Seasonal events–a grand market that sets up near the capital for two weeks at summer’s end, perhaps, or a tournament each midwinter with a large purse as grand prize–could give some choices with actual weight about what paths to pursue to your players, as well as force them to be reasonable.

Ticking off days on a calendar also makes it easier to remember to track things like consumption of rations or progression of diseases or other day to day concerns, and helps keep the world from being in endless midsummer, or jumping seasons arbitrarily from session to session simply for convenience.

Plenty of crpgs have calendars and track the passage of months and years. Ultima springs to mind, with its moongates having different destinations based on the phase of the moon–this was even the only way to reach one of the plot-critical locations in several games. The Might and Magic series tracked the ageing of characters and had seasonal events only available on certain dates. Seiken Densetsu 3 tracked the weekdays and they affected spell power. World of Warcraft may or may not count, since it literally uses our calendar 1:1 without renaming anything, but it makes use of seasonal and timed events as well. Persona 3 and 4 have gameplay elements directly and thoroughly tied to the calendar as well–in spite of simply using a real world calendar, I count it here because of how that is used in game, since similar uses would and have been applicable in fantasy calendars as well.

I definitely agree that all this is not necessary for every campaign, however. Just as many DMs don’t care about tracking encumbrance or enforcing that players track their food and ammunition as adventures progress, this level of detail is not always needed. Each group must make their own choices on how immersive they want to make their campaign world. This series of articles strives to provide starting points and resources for those who want to expand their campaign settings.

That said, there’s absolutely such a thing as trivial details that only a video game can or should track. You won’t see a WB101 article about currency, for example, because while it can seem to add a good deal of immersion to create different coins and currenies between kingdoms, with slightly different and fluctuating values, the actual practice of doing so bogs down the game when it comes time to spend money–”Okay so I have fifteen Florinian guilders, ten Guilderian florins, seventy five Tarnan sovereigns, and a couple of Shapeirian dinars…it says this is 100 gold to buy, is that enough?” “Well Tarna’s going through an economic crisis since the demon invasion’s cut off access to their gold mines, so their currency is through the floor…”

Not worth the effort. Calendars, on the other hand? Easy, and judging by the google results, far from out of place in this series of articles.