I suppose theoretically you could lay it any time, but now we've moved into the first frosts of autumn it's not really advisable. Newly laid turf needs to be watered regularly to encourage the roots to establish themselves and if you're watering it when the ground is frosty your turf will not like it. If however, it's just a case of increasing a small lawned area and only involves a few extra turves it's possible worth a risk. Otherwise, it's probably better to wait until the Spring.