The mileage and one-third (1/3) of the salary for a regular session shall be paid at the beginning of the session. After thirty (30) days of the session have expired, another one-third (1/3) of the salary shall be paid. The remaining one-third (1/3) of the salary shall be paid (a) on the last Friday before the ninetieth day of a ninety-day session, or (b) on the earlier of the last day the Legislature is convened or the last Friday before the one-hundred-twenty-fifth day of a one-hundred-twenty-five-day session, as the case may be.