D.C. Mun. Regs. tit. 11, r. 11-3110
In computing any period of time specified in this chapter, calendar days shall be counted.
In computing any period of time specified in this chapter, the day of the act, event, or default after which the designated period of time begins to run shall not be included. The last day of the period so computed shall be included unless it is a Saturday, Sunday, or official District of Columbia holiday, in which event the period shall run until the end of the next day that is neither a Saturday, Sunday, nor official holiday.
Whenever a party has the right or is required to do some act within a prescribed period after the service of a notice or other paper, and the paper or notice is served upon the party by mail, three (3) days shall be added to the prescribed period.
Except as otherwise provided by law, whenever an act is required or allowed to be done at or within a specified time, the time fixed or the period of time prescribed may, for good cause, be extended or reduced by the Board with notice to all parties.
D.C. Mun. Regs. tit. 11, r. 11-3110