The last frost date in the spring and the first frost date in the fall are estimates that rely on historical data for a particular zone or micro-climate. The erratic spring temperatures with both early and late arrivals of spring and fall, over the last decade, are useful examples of why these dates cannot be relied upon exclusively to judge safe days for planting or dormancy.  A good eye to the weather forecast and common sense are essential for a gardener.

With that caveat, there are some useful resources available to estimate the first frost date for an area. Cornell University Department of Horticulture publishes a map of Average Last Spring Frost Dates and Average First Fall Frost Dates for New York State.  Absent a significant microclimate effect, the first fall frost for Manhattan is on average after November 10th and for most of the Bronx, between October 30 and November 10th. Other weather-related statistics and information for New York State are available from Cornell on their Weather Gardening Resources page.

Courtesy of the NYBG Plant Information Service