Whenever the final date prescribed in any of the regulations in this title for the performance of any act falls on a Saturday, Sunday, national holiday, State holiday on which the office of the county or State Farm Service Agency committee having primary cognizance of the action required to be taken is closed, or any other day on which the cognizant office is not open for the transaction of business during normal working hours, the time for taking required action shall be extended to the close of business on the next working day. Or in case the action required to be taken may be performed by mailing, the action shall be considered to be taken within the prescribed period if the mailing is postmarked by midnight of such next working day. Where the action required to be taken is with a prescribed number of days after the mailing of notice, the day of mailing shall be excluded in computing such period of time.