formed the first complete calendar in the Reik basin to include every religious ceremony and cultural festival while also being accurate to the times of planting and harvest.
It should also be noted that Sigmar did not create a calendar that includes every ceremony and cultural festival. He created a position by the name of the Master of the Calendars or whatever that was in charge of that stuff, but the Calendar was taken from the Dwarves.
The planet has 400-day year and a 25-day lunar cycle. This should make the calendar blindingly obvious. Unfortunately:
"We have a five-day week based on a 25-day lunar month," says the only sane tribe.
"We have a fourteen-day week based on the maximum time we can withstand a siege," says another.
"We have a seven-day week with each day dedicated to a different God."
"We have a four-day week and a 16-day month, but we don't remember why."
"We have an eight-day week because Ulric says so."
"Are any of you willing to compromise?" asks Sigmar.
"Fuck no," says literally everyone.
"Okay, you, find a way to make a compromise solution that everyone likes," says Sigmar to some poor bastard.
"HAIL TZEENTCH," says that guy pretty much instantly, founding a minor Cult that exists to this day.
"Fuck it," says Sigmar, "I'll just copy the Dwarves."
"Our original calendar had twelve 33-day months, with four non-month days on the solstices and equinoxes," say the Dwarves. "Unfortunately the existence of Morrsleib means we now have ten 33-day months, two 32-day months, four non-month days on the solstices and equinoxes, one non-month day on the new year for one of the days when Morrsleib is full, and one non-month day at the completely unpredictable time throughout the year for the other day when it's full."
"I don't even care any more," says Sigmar, and adopts it.