Whenever you're able to achieve a friable, crumbly surface on the top of the soil, really - if its really wet and sticky, you won't be able to achieve that, but any time from now on is good.

I thought the old wives tale said that grass seed shouldn't be sown when there's an R in the month - so I would have said May, June, July, August ...... so no chance of frost :)

... I could be wrong though......

plus I'm in NE Scotland so that's probably more applicable than the south of the country.....

Blimey, never heard that before Donna - must be exclusive to your part of the world. Throughout most of the UK, grass seed can be sown between March and October, though further north, late March and early October.