When the last day prescribed under authority of the internal revenue laws for performing any act falls on Saturday, Sunday, or a legal holiday, the performance of such 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 purposes of this section, the last day for the performance of any act shall be determined by including any authorized extension of time; the term “legal holiday” means a legal holiday in the District of Columbia; and in the case of any return, statement, or other document required to be filed, or any other act required under authority of the internal revenue laws to be performed, at any office of the Secretary or at any other office of the United States or any agency thereof, located outside the District of Columbia but within an internal revenue district, the term “legal holiday” also means a Statewide legal holiday in the State where such office is located.
26 U.S. Code § 7503 - Time for performance of acts where last day falls on Saturday, Sunday, or legal holiday
Applicability of This Section for Purposes of Section 10222(b) of Pub. L. 100–203
“Section 7503 of the 1986 Code shall apply for purposes of determining whether any disposition meets the requirements of section 10222(b)(2)(B) of the Revenue Act of 1987 [Pub. L. 100–203, set out as a note under section 301 of this title]. If any disposition meets the requirements of such section by reason of the preceding sentence, for all purposes of the 1986 Code, such disposition shall be deemed to have occurred on December 31, 1988.”