A Tortoise has 13 large patterned squares in the centre for the thirteen full moons of the year.
There are twenty-eight smaller squares around the perimeter of shell, representing twenty-eight days of each lunar month.
28 days = 4 weeks
4 weeks= 1 lunar month
12 months = 52 weeks
52 weeks divided by 4 (4 weeks in 1 month) = 13 months.