According to the NY Consolidated laws (Chapter 17-B, Article 3), a holographic will is valid in NY ONLY if the following apply:
"Sec. 3-2.2 Nuncupative and holographic wills.
(b) A nuncupative or holographic will is valid only if made by:
1. A member of the armed forces of the United States while in actual military or naval service during a war, declared or undeclared, or other armed conflict in which members of the armed forces are engaged.
2. A person who serves with or accompanies an armed force engaged in actual military or naval service during such war or other armed conflict.
3. A mariner while at sea."
Source:
http://assembly.state.ny.us/leg/?cl=38&a=9