N.Y. Comp. Codes R. & Regs. tit. 20, § 152.3
Tax Law, § 691(c)
When the last day prescribed in this Subchapter (including the last day covered by an extension of time) for filing a New York State income tax return falls on a Saturday, Sunday, or a day which is a legal holiday in New York State, see section 2399.3 of this Title.