Last week, I brought up the thorny topic (to me) of calendars. To me, our month and day names stand out as products of our world, so they don't work well in a fantasy setting. (Exceptions would be alternate earths or the stealth fantasy-setting-that-is-actually-scifi that used to be popular: colonists settled the planet long ago, but the origins have long since been lost and it reads like a fantasy realm.) One easy solution is to simply "reskin" our months and days with new names.
Otherwise, the challenge is to make a system that is 1) Logical and usefully divided. People rely upon the pattern of weeks to order their lives. A twenty day week would be unwieldy. 2) Intuitive. Throwing a foreign system at the reader, it needs to be easy to pick up. 3) Roughly equivalent in the length of a year. Extra days or decreased days can add up to characters who aren't quite as old as they say they are ...
The easiest layout is to shorten the months to 28 days. Then you have thirteen of them, and exactly four weeks. In Unnatural Causes, Pinnacle - a day of rest - is in the middle of a nine day week. The week is counted down and up from Pinnacle. It comes out easy to follow, once you wrap your brain around the fact that three-before is followed by two-before.
Again, intuitive is the key. If the writer dumps a lexicon of month / week / day names on the reader, the story grinds to a halt ... and it probably doesn't serve the intended purpose, as information overload leads to skimming.
But if it all seems to make sense and the reader can track how much time has passed, then the calendar has served its purpose. Like most of the iceberg in worldbuilding, the reader will (hopefully) feel the structure without needing to see it.
Word count this week: 2,995
Pages edited: 22 (yes, really)