Typically, bluegrass lawns will enjoy the cooler temperatures and more plentiful rainfall at this time of the year. NOW is the best time to eliminate dandelions and other broadleaved weeds (use weed and feed or spot treat with your favorite approach) while the weeds are actively growing. It also eliminates weeds for next spring too.
Lawn fertilizer is very welcome at this time of the year, and will help to beef up your turf and get it looking great. Whether you use chemical or organic fertilizers, if you only use fertilizer one time per year...this is that time!
Eliminate your weeds, feed your grass, and fill in those bare spots. Now is the time for new lawn seeding also.