OK to plant 3 x 12 foot new grass area in 80 degree weather if watered often?

I have successfully seeded a whole yard in 80 degree weather. It did require at least two waterings per day until the roots were well established.

By the way, most grass is a warm weather plant.

Better off if you wait until autumn. Grass is a cool weather plant.

If you want to try it anyway: well, you won't be spending too much money on seed for such a small area. Plan on watering it twice a day, and overseeding in the autumn. You won't have total success in the summer - but maybe not a total failure, either.

Its best to water any type of plant in the morning or in the evening, the plant will be able to better soak up the water. If you water it during the hottest part of the day, it will evaporate more quickly and it will be less efficient.