If you are planning to core-aerate your tall fescue or Kentucky bluegrass lawn this spring, March or early-April is the time. Coring early in the spring gives cool-season lawns a chance to recover before crabgrass and other warm-season annual weeds start to germinate.
Core-aerating is one of the best things you can do for your lawn.
It relieves compaction, hastens thatch decomposition, increases water infiltration and helps promote better root growth. Pay attention to the soil moisture level when coring. The soil should easily crumble when worked between the fingers. If it is too wet, the machine’s tines will plug and it will merely punch holes in the wet soil, which increases compaction. If it is too dry, the tines will not be able to penetrate deeply enough to be of benefit. (Ward Upham)