D.C. Mun. Regs. tit. 17, r. 17-1617
In computing any period of time specified in this chapter, calendar days shall be counted unless otherwise indicated.
In computing any period of time specified in this chapter, the day of the act, event, or default from which the designated period of time begins to run shall not be included.
The last day of the period shall be included unless it is a Saturday, Sunday, or legal holiday, in which event the period runs until the end of the next day which is not a Saturday, Sunday or legal holiday.
Whenever a party has the right or duty to act or proceed within a prescribed period after the service of a notice or other paper upon the party and the notice or paper is served upon the party by first class mail, three (3) days shall be added to the prescribed period.
D.C. Mun. Regs. tit. 17, r. 17-1617