The RHS website explains: “Germination should take between seven and 10 days and seedlings will become established before the first severe frosts in late autumn, this is the preferred time as the roots establish better through the winter.
“Sowing seed in spring, grass will want to flower quite early before roots have established.”
The BBC’s Gardening Guide for growing grass explains the best time to sow grass seed is late summer to mid-autumn.
However, in lieu of this, mid-spring is also an optimum time to grow grass.
Just make sure you give the new grass plenty of water to allow it to flourish.