N.Y. GCT Law § 71

§ 71. Mailing rules; holidays.--(a) Timely mailing.--If any claim,\nstatement, notice, petition, or other document (including to the extent\nauthorized by the administrator, a return or a declaration of estimated\ntax) required to be filed within a prescribed period or on or before a\nprescribed date under authority of any provision of this local law is,\nafter such period or such date, delivered by the United States mail to\nthe administrator, bureau, office, officer or person with which or with\nwhom such document is required to be filed, the date of the United\nStates postmark stamped on the envelope shall be deemed to be the date\nof delivery. This subdivision shall apply only if the postmark date\nfalls within the prescribed period or on or before the prescribed date\nfor the filing of such document, determined with regard to any extension\ngranted for such filing, and only if such document was deposited in the\nmail, postage prepaid, properly addressed to the administrator, bureau,\noffice, officer or person with which or with whom the document is\nrequired to be filed. If any document is sent by United States\nregistered mail, such registration shall be prima facie evidence that\nsuch document was delivered to the administrator, bureau, office,\nofficer or person to which or to whom addressed. To the extent that the\nadministrator shall prescribe by regulation, certified mail may be used\nin lieu of registered mail under this section. This subdivision shall\napply in the case of postmarks not made by the United States Post Office\nonly if and to the extent provided by regulations of the administrator.\n (b) Last known address.--For purposes of this local law, a taxpayer's\nlast known address shall be the address given in the last return filed\nby him, unless subsequently to the filing of such return the taxpayer\nshall have notified the administrator of a change of address.\n (c) Last day a Saturday, Sunday or legal holiday.--When the last day\nprescribed under authority of this local law (including any extension of\ntime) for performing any act falls on Saturday, Sunday, or a legal\nholiday in the state of New York, the performance of such act shall be\nconsidered timely if it is performed on the next succeeding day which is\nnot a Saturday, Sunday or a legal holiday.\n