Add the personal year number to the number of the calendar month and reduce. January is 1 and December is 12, which reduces to 3 before it is added. The result runs 1 to 9.
Meaning
The shorter cycle inside the personal year, used for the finer timing that a year number is too coarse to give: which month of a four year to start building, which month of a nine year to let go. Modern practice reads it as the month's weather within the year's season.