I have a couple of small bare areas in my lawn that I did not get seeded last fall. September is by far the ideal time to seed bluegrass or tall fescue.
Spring is acceptable but more problems can emerge with weeds and summer stress.
Mid March through early April would be the best time for establishment. Be sure to prepare the soil for a good stand. Small areas can be hand raked while larger areas may need mechanical equipment such as a verticut or core aerator. Avoid the use of crabgrass preventers in the areas that have been seeded. Tupersan, a preemergent herbicide, will provide some relief from the crabgrass competing with the new stand of grass.