This is known as dormant seeding. You put the seed down after it is too cold to germinate and the snow melt and freeze/thaw cycles during the winter help to get seed-to-soil contact and it germinates in the spring when it is warm enough with the natural spring moisture.

The first snow is probably a good time to do it because the melting snow will help bring it into contact with soil. In some areas, people wait until after the new year to do it. Most years around here, that wouldn't work because once we get a significant snowfall, there's snow cover until April or May.

Ah right thanks, so would it be worth it in NJ then? Cos once it snows here the snow stays down for awhile?

I'm not too far from you and I plan on dormant seeding around February or so. I did this last year with some short-term success although my KBG seedlings didn't make it past a 1-2 week long drought we had (I don't irrigate). The freezing and thawing cycles towards late winter seem to ensure good seed to soil contact. In fact, I couldn't find one seed on the ground as they were all "melted" just under the surface. Of course this is all weather dependent but our snow cover doesn't seem to linger too long unlike some areas, like Utah as bpgreen states.

The thing about seeding late and having snow cover over the seed is all about not having to think about when should you overseed in the spring.

It all comes down to though, temperature....and not warm March days that might show up for a day or so....its not air temperature that tells the seed to start germinating.....its the soil temperature. Above 40º - 45º minimum is usually the temperature is has to rise to.

Until the sun shines and the air temperature sticks around for sufficient time to raise the soil temp, then the seed will just sit there....and wait until the soil temperature is most opportune.

The thing too about seeding in the late fall is that the birds wont likely be around to snack; they've packed their bags.
The ones that do stick around.....well, heck, be kind and let them have a seed or two.
They say its not a good idea to feed birds in the winter--they have to fend for themselves in hard times so they know better where to find food.
Make it too easy and you had better be there in those hard times.

lawndivot...why would you overseed a KBG lawn? Are you not seeing as much spreading as you would like?

Thanks, i think the time is coming closer to dormant seed in NJ.

What sead should i use