N.Y. Comp. Codes R. & Regs. tit. 9 § 4111.18
All nominations not actually received at the hour of closing shall be ineligible, except those by letter bearing postmark not later than noon of the following day (omitting Sunday) or entries notified by telegraph, the telegram to be actually received at the office of sending at or before the hour of closing, such telegram to state the color, sex, and name of the horse, the class to be entered; also to give the name and residence of the owner and the party making entry. Whenever a nomination or payment in a stake, futurity, or early closing race becomes payable on a Sunday or a legal holiday that falls on Saturday, such payment is to be due on the following Monday and if made by mail the envelope must be postmarked on or before noon of the following Tuesday. If a payment falls on a Monday that is a legal holiday, such payment is due on Tuesday, and if made by mail must be postmarked on or before noon of the following Wednesday.
N.Y. Comp. Codes R. & Regs. Tit. 9 § 4111.18