D.C. Mun. Regs. tit. 16, r. 16-1505
In computing the periods of time under this chapter, the day of the act or event shall not be counted and the last day of the period shall be counted, unless it is a Saturday, Sunday, or legal holiday, in which event the time period continues until the next day which is not a Saturday, Sunday, or legal holiday.
When the period of time for computation is less than seven (7) days, Saturdays, Sundays, and legal holidays shall not be included in the computation of time.
Legal holidays shall include New Year's Day, Martin Luther King Day, Washington's Birthday, Veteran's Day, Thanksgiving Day, Christmas Day and any other day appointed as a holiday by the President or the Congress of the United States or by the Mayor of the District of Columbia (on the actual day the legal holiday is celebrated by the government of the District of Columbia).
Whenever a party has the right or is required to do some act within a prescribed period after the service of a notice of paper upon that party by mail, three (3) days shall be added to the prescribed time period.
D.C. Mun. Regs. tit. 16, r. 16-1505