Go to Profile > settings > payroll > pay run schedules
Click on edit on one of the available schedules
Choose your first payroll date for this schedule: your payroll date is the day when you'll process your pay runs to generate payslips. Hence, your first payroll date is the first day during which you'll process the pay run for this schedule.
Choose your first payment date: your payment date is the payday. Therefore, the first payment date is the day when you will pay your employees for the first time for this new schedule.