When the last day prescribed administered by the Office of Tax and Revenue for performing any act falls on Saturday, Sunday, or a legal holiday, the performance of the act shall be considered timely if it is performed on the next succeeding day which is not a Saturday, Sunday, or a legal holiday. For the purposes of this section:
(1) The last day for the performance of any act shall be determined by including any authorized extension of time.
(2) The term “legal holiday” means a legal holiday in the District of Columbia.