Scarifying your lawn is a particularly stressful process for grass. This is why you need a proper plan on how you plan to take care of your grass after scarification.
It is common to find that your lawn looks awful after scarification, but that is nothing to be worried about - in a few weeks your lawn will be looking better than it ever has.
Your aim after scarifying your lawn is to promote rapid, healthy grass growth, and that is why when you do it, and the conditions you do it in are so important.
Scarification is best done when the weather conditions promote grass growth. This is when the temperature is above 6 Celsius, because the ground has enough moisture for the grass to feed from, and there is plenty of sunlight.
It is best to scarify your lawn more deeply in the Autumn, as you will be removing more thatch and moss from your lawn therefore exposing more of the ground. This leaves it open for weeds etc. to grow in.