A good rule of thumb is after Mother's Day in Central NJ. I'm near New Brunswick.

I live in northeast coastal monmouth county and rarely do I have a frost after april 1 although the soil and air are still too cold for tropicals

I'm in Cherry Hill, and I remember my walls-o-water over the tomatoe seedlings having snow on them the first week of April one year. (The seedlings survived just fine.) We usually have a cold snap during the first 2 weeks of May, but weather.com gives us the 10 day forecast, so you can make your own judgement on when to plant annuals.

Hi, this site gives April 3 for the frost date, earlier than the date I usually see.
For Newark NJ, in the northern/central part of state.

Keep in mind the micro-climate can have as much as a 10-degree or more difference. For ex I garden in a community garden which is wide open to the winds, and the last frost occurs there later than in the community in which I live.

If your plants are sheltered, for example by your house, if they are on the eastern/southern side of the house, can considerably raise the temp.

Here is a link that might be useful: Frost date NJ