From when can I sow grass seeds on my lawn without risk of frost?

Dear Mr. R., lawns grow best from a ground temperature of 14 °C, as they can then germinate in optimum conditions. Should you sow them any earlier, the seeds will lie around unnecessarily long, until they reach a temperature suitable for germination. The frost itself is not a major risk for the young shoots, but rather the frost dryness.
 
The plant is unable to draw in water from the frozen ground, and dries out. Therefore, the topic of frost damage to freshly-sown lawns tends to be a problem in autumn and towards winter, rather than a spring issue. This is of course excepting the really severe, prolonged periods of late frost.