D.C. Mun. Regs. tit. 20, r. 20-4309
This section applies to all periods of time prescribed or allowed by the Hazardous Waste Management Regulations, chapters 42 and 43 of this title.
In computing any period of time measured in days or calendar days, the day of the act, event, or default from which the designated period of time begins to run shall not be included.
For any period of time that is measured in days or calendar days, the last day of the period shall be included, unless it is a Saturday, Sunday, or a legal holiday. When the period of time prescribed or allowed is less than eleven (11) days, intermediate Saturdays, Sundays, and legal holidays shall be excluded from the computation, unless the period of time is measured in calendar days.
Whenever a person has the right or the obligation to do some act within a prescribed period after the service of an order or other paper upon the person, and the order or other paper is served by United States mail or third-party commercial carrier, five (5) calendar days shall be added to the prescribed period, unless a statute provides otherwise.
D.C. Mun. Regs. tit. 20, r. 20-4309