It is safe to plant grass seed after a hard frost, as long as it's the last frost. Pay attention to weather cycles for your region or consult your local county extension office for general guidelines for frost dates in your area.
If a late frost is forecasted after grass seeds are planted, protect them with a proper cover. Agri-Fabric, a floating cover, is one such shield that will guard the seeds against frost.
Landscape contractor Roger Cook of "This Old House" magazine suggests early fall as the ideal time to plant grass seed. During this time, there's less competition for weeds, and the soil is at the ideal temperature for germination.