According to a recent Nassau County Case (Olszewski v. Neuman) - A landlord must supply heat once October begins, but is not obligated from May through September.
While this Court made this specific decision based upon its individual facts, it is estimated that the weather during a given year would be applicable to a proper determination of a future case. Also, this decision is not from an Appellate Court and does not constitute precedent that is binding into the future.
Nonetheless, a great black line for those who dabble in the business - Get that heat on before October!!!