When is it? In New England, that's traditionally been Memorial Day. That's way off. Furthermore, global warming is moving that date up. The USDA is working on new plant hardiness zones. That's still too general. I want to know the last frost day in my town and even my backyard.
Weather Underground. You can search for Personal Weather Stations near your zip code. Under the radar screen you should be able to find the icon on the right called Weather Stations. You can click on them and see maps with the location. Try to find one closest to your backyard.
You want a station with at least 1 years worth of data so you can find the last time in April-May when the temp went below 34 degrees. It might take some doing but data is out there.
If you really want to know the weather in your backyard, I think Weather Underground should be able to point you in the right directions for a weather station setup.
That's what I did to estimate last year's frost. For this year, I set up my own recording thermometer.
I'm a computer geek. I have an old PC running Linux that's on 24x7 anyways. I decided to use something called Dallas 1-wire.
I got the hardware from Hobby Boards. A DS9490R-A plugs into a USB port ($28), an R11 to RJ45 cable (if you can't make your own) ($6), an RJ45 to RJ45 ($1.25) to connect a longer cable and a temperature sensor ($18). Plus a standard cat 5/6 network cable. For $54 you have all the hardware for a recording thermometer for less then the cost of a weather station. You can connect several temperature sensors along a wire. I have one inside right next to my computer instead of the RJ-RJ.
For the software, I run Linux on the PC. I'll use Fedora or Ubuntu which you can freely download and use. I've setup a 1-wire thermometer on a cast off PC w/ 128 MB RAM and a 6 GB hard drive.
To talk to the 1-wire net, I use owfs and the owhttp driver. To record and graph the temperatures of my sensors, I use Thermd. If you're interested in temperature sensing, look around Dan's site. He has a good list of different temperature sensing setups that may work better for you then my setup.
Digitemp is another set of software that can record and graph 1-wire temperatures. I'm not sure it supports as many 1-wire devices as the owfs setup.
If you're going to put the temperature sensor in the garden (and that's the point isn't it?) you need to house it somehow. The "proper" way is to use a pagoda to lessen the effects of direct sunlight. I used a stack of plastic bowls with a hollow in the middle. The sensor is inside the hollow.
Last frost in Spring 2009
First frost in Fall 2009
I did this last year so I would have a recording of frost dates for last year. Boy, that's lots of work just to get frost dates!
The 1-wire setup is easy to extend. For $18 + a pagoda + a network wire, I get another temperature. I'm thinking of something to measure soil temp, a cold frame/green house, etc....