Day Factor is a required field when setting up a Payroll Group.  

The Day Factor is the number of working days in the year for your payroll group, and is used when calculating an employee's hourly rate from their basic salary.  An employee's hourly rate is used when the employee works overtime, or when his salary is deducted due to unpaid absences or tardiness.  For a five-day work week, the day factor is around 260.  For a six-day work week, the day factor is around 313.