Has the grass seed germinated, or started to germinate? If so, it will need to be irrigated regularly (to avoid drying out for too long) while the new plants establish, as they are not tolerant of drought. If no seed has germinated, then watering beginning now might get them to sprout and mature enough to survive the winter, but it's hard to predict; it wouldn't hurt to try, since the germination rate would probably be lower if the seed survived the winter and sprouted in spring. (In part because birds and rodents will probably eat some of the exposed seed in the meantime.)

While current mild temperatures are certainly mild enough to support continued grass growth, the shortening days are rapidly approaching the point at which few plants would be able to photosynthesize enough to keep growing, so things will then stall until spring. Either way, you might need to overseed one more time in spring (weather-depending, perhaps mid-March or so) if this attempt results in more sparse turfgrass than expected. Our Care and Maintenance of a Lawn After Seeding web page provides irrigation tips and other information that might be useful.

Miri