Why include a 13 month calendar?

In a primitive environment this is a simple reusable calendar the same printout can be used year after year. This calendar was in use before things got unnecessarily complicated when we adopted the current Georgian calendar.  Each month is the same length and starts on the same day of the week.  The moon cycles can be used to keep time.  I am not recommending its use.  Only that it can be used if needed.