New York Consolidated Laws, General City Law - GCT § 71. Mailing rules;  holidays

(a) Timely mailing.  If any claim, statement, notice, petition, or other document (including to the extent authorized by the administrator, a return or a declaration of estimated tax) required to be filed within a prescribed period or on or before a prescribed date under authority of any provision of this local law is, after such period or such date, delivered by the United States mail to the administrator, bureau, office, officer or person with which or with whom such document is required to be filed, the date of the United States postmark stamped on the envelope shall be deemed to be the date of delivery.  This subdivision shall apply only if the postmark date falls within the prescribed period or on or before the prescribed date for the filing of such document, determined with regard to any extension granted for such filing, and only if such document was deposited in the mail, postage prepaid, properly addressed to the administrator, bureau, office, officer or person with which or with whom the document is required to be filed.  If any document is sent by United States registered mail, such registration shall be prima facie evidence that such document was delivered to the administrator, bureau, office, officer or person to which or to whom addressed.  To the extent that the administrator shall prescribe by regulation, certified mail may be used in lieu of registered mail under this section.  This subdivision shall apply in the case of postmarks not made by the United States Post Office only if and to the extent provided by regulations of the administrator.

(b) Last known address.  For purposes of this local law, a taxpayer's last known address shall be the address given in the last return filed by him, unless subsequently to the filing of such return the taxpayer shall have notified the administrator of a change of address.

(c) Last day a Saturday, Sunday or legal holiday.  When the last day prescribed under authority of this local law (including any extension of time) for performing any act falls on Saturday, Sunday, or a legal holiday in the state of New York, 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.