27 CFR 771.28: Computation.

In computing any period of time prescribed or allowed by this part, the day of the act, event, or default after which the designated period of time is to run is not to be included. The last day of the period to be computed is to be included, unless it is a Saturday, Sunday, or Federal holiday, in which event the period runs until the next day that is not a Saturday, Sunday, or Federal holiday. Pleadings, requests, or other papers or documents required or permitted to be filed under this part must be received for filing at the appropriate office within the time limits, if any, for such filing.