Depends why you're doing it!
If you are overseeding what is there, you would weed and feed it first. Ensure the existing grass is cut well. rake or scarify to remove any dead material/moss etc. Aerate it - most important (spiking all over the lawn at close intervals) then mix the grass seed you want with fine compost or loam and brush it in. Wait for the rain and try to stay off it. It is a little late to do this now - sept/oct is best.
If you want to replace it completely - wait till summer, spray it off, then prepare the ground and re seed in september.
But it really depends why it needs doing - is it poor soil, moss, shady, wet, tree root problems ???

Bit late now, as grass needs 5 degrees plus to germinate, and fluctuating temperatures, (plus overnight lows of near freezing, despite daytime highs of14 degrees plus) won't aid germination. Phil J