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In Egalia, each month consists of 28 days based on wom’s menstrual period, and a year consists of 13 months. (28*13=364) 

The 13th month is called Donnuary. It's named after Egalia's god Donna Jessica. They have Grand Menstruation Games in the 13th month to celebrate and honor the annual harvest and the achievements of the Egalians.

And since each week consists of 7 days, each month can be perfectly divided in four weeks.  The layout the calendar remains identical every month.
Therefore, the concept of the calendar in Egalia is more like a sheet of paper with a template that can be applied to every month. And this leads to the invention of the erasable calendar that saved an enormous amount of wood being cut.