April is a good time to plant grass because the spring growing season is just getting underway. If you have an early warm up, you can plant it in March; if it's a cooler spring, May is OK. Cool season grasses also do well if they are planted in the fall -- between Aug. 15 and Sept. 15.
Take the temperature of the soil. Cool-season grass does best in 50 to 65 degrees F. Warm-season grass seed needs soil that is 65 to 70 degrees F. Northerners can get away with sowing seed at least 90 days before the first expected frost. Southerners should plant grass at least 60 days before the first frost.
It's possible to grow grass in late fall, and have it sprout the following spring. The danger with doing this is that the snow and wind may move the seeds, creating gaps in the new lawn.