Climate Central has your answer with its post The First Frost is Coming Later. They provide graphs, like the one here for NYC (about 20 days later since 1970), for most major cities in the U.S. They don’t provide the data, but you can try and send them an email and they may send it to you. Alternatively, this could be a great stats project where students get the data themselves for a city of their choice and create the chart. You can get weather data from NOAA Climate Data Online.