Payroll remittance due dates are always based on when an employee is paid for his or her services (payday) rather than the pay period for which the services are rendered.
For example, if a pay period ends on March 28 but the employee get paid for this period on April 4, the remittance due date would be determined based on April 4, the day you paid your employee. For regular remitters, the due day is May 15. It is not April 15 even though pay period ends in March.