Fantasy
Invented calendars only start earning their keep when the month names do some actual work.
If a festival, a debt due date, a harvest rite or an old superstition depends on “the third moon of Ashfall”, then the calendar has weight. If it’s just decorative flavour, it tends to sit there looking pleased with itself. Which is fine, but not very useful.
The interesting bit is when people in the story have to live by it. Miss the wrong month and you miss the levy, the saint’s day, the safe sailing season, the one week when nobody will cross the river. That’s when the calendar stops being worldbuilding wallpaper and starts behaving like part of the plot.
If a festival, a debt due date, a harvest rite or an old superstition depends on “the third moon of Ashfall”, then the calendar has weight. If it’s just decorative flavour, it tends to sit there looking pleased with itself. Which is fine, but not very useful.
The interesting bit is when people in the story have to live by it. Miss the wrong month and you miss the levy, the saint’s day, the safe sailing season, the one week when nobody will cross the river. That’s when the calendar stops being worldbuilding wallpaper and starts behaving like part of the plot.
Invented calendars can do a bit more than schedule things. They can show who has power over time. If the ruling class names the months, or changes the festival dates, that says something ugly very quickly. Also, a calendar can be social pressure in disguise. People miss work, weddings, funerals, whatever, because they’re on the wrong system. That’s more interesting than another fake month with a nice sound to it.