It depends on the configuration of the loan category. Some credit unions require one payment per period; others allow the mortgage to be paid ahead a certain number of periods (up to 9 max.) or even unlimited periods.
NOTE: Even if you don’t allow members to pay ahead, a payment CAN be made at any time during the month leading up to the due date. For example, if a payment is due June 1, a member can make their payment any day during May.