It depends on the temperature not any specific date. It has to reach 57 degrees for weeks without falling below before the larvae develop to the right stage to infect dogs.
Since (most) heartworm medications destroy (not prevent) heartworms you give them 30 days after the start of the 57 degree weather and continue until 30 days after the end.
I leave room for error and make it 53 degrees just to be on the safe side but around here we jump from cold to hot weather practically overnight.
Heartworm | B-Naturals.Com Newsletter
"An interesting and important factor about the time the larvae is evolving in the mosquito is the temperature. The temperature that the mosquito is living in must not go below 57 degrees during this stage. A quote from Langsley Russell’s article states: “The importance of temperature: While the larvae are developing in the mosquito, development continues only when the temperature is above 64 degrees F. Further, the temperature MUST remain above 57 degrees Fahrenheit at all times, day and night during the entire mosquito cycle. If at any time during the development into the L3 stage, the temperature drops below 57 F, the development is aborted and must start over. Remember, it is only the L3 larvae which are capable of infesting your dog.”
Testing....
Heartworms can be detected by blood test. The filtration test finds microfilariae in the blood; the occult test locates adult worms in the heart. Many veterinarians prefer to do both tests as the absence of microfilariae in the blood does not necessarily mean that there are no adult worms in the heart. Both tests are done with a single blood draw, preferably in the early spring before daily temperatures warm above 57 degrees F.
Dog Owner's Guide: Heartworm Disease
If you want to find more info just do an online search for Heartworm and 57 degrees.