D.C. Mun. Regs. tit. 16, r. 16-1416
In computing the periods of time under this chapter, the day of the act or event from which the time period begins to run shall not be counted. The last day of the period shall be included unless it is a Saturday, Sunday, or legal holiday, in which event the period continues to run until the next day that is not a Saturday, Sunday, or legal holiday.
When the period of time is less than seven (7) days, Saturdays, Sundays, and legal holidays shall not be included in the computation time.
Legal holidays shall include those set forth in D.C. Code § 28-2701 et seq., (1996 Repl. Vol.), or any other day designated as a legal holiday in the District of Columbia.
Three (3) days shall be added to the prescribed time period whenever a party is served a notice or document by mail.
D.C. Mun. Regs. tit. 16, r. 16-1416