The SBA’s forgiveness worksheet (which has been posted by Paulie here) says (emphasis mine)
Eligible payroll costs. Borrowers are generally eligible for forgiveness for the payroll costs paid and payroll costs incurred during the eight-week (56-day) Covered Period (or Alternative Payroll Covered Period) (“payroll costs”). Payroll costs are considered paid on the day that paychecks are distributed or the Borrower originates an ACH credit transaction. Payroll costs are considered incurred on the day that the employee’s pay is earned. Payroll costs incurred but not paid during the Borrower’s last pay period of the Covered Period (or Alternative Payroll Covered Period) are eligible for forgiveness if paid on or before the next regular payroll date.
So the eight weeks starts with wages incurred on the start of the Covered Period. You can take eight weeks-plus-the-next-payday to pay them.