31 C.F.R. § 306.15
(b) Closing of transfer books. The transfer books are closed for one full month preceding interest payment dates and call or maturity dates. If the date set for closing of the transfer books falls on Saturday, Sunday, or a legal holiday, the books will be closed as of the close of business on the last business day preceding that date. The books are reopened on the first business day following the date on which interest falls due. Registered securities which have not matured or been called, submitted for transfer, reissue, and coupon securities which have not matured or been called, submitted for exchange for registered securities, which are received during the period the books for that loan are closed, will be processed on or after the date such books are reopened. If registered securities are received for transfer, or coupon securities are received for exchange for registered securities, during the time the books are closed for payment of final interest at maturity or call, unless otherwise provided in the offering circular or notice of call, the following action will be taken:
[38 FR 7078, Mar. 15, 1973, as amended at 64 FR 38125, July 15, 1999]