There are 52 weeks in the year which is 13 sets of 4 so if your parents simply pay for 4 weeks at a time them you need to let them know when it is payday and it won't follow the first of the month. Alternately you add the extra onto each month so the cost per month is more than 4 weeks of fees. The average month has more than four weeks based on part weeks at the beginning and end of the calendar and that is how you get the equivalent of 13 pay periods per year.