The best time is autumn, but it can be done in March. It'll just look rough for a while, but it will recover. Cut before and after you've done it though.

I do it after the first spring mow to get up a lot of the over winter moss and rubbish. I then apply lawn weed /moss killer with a good feed and scarify again about six weeks after the application. I repeat this after about eight weeks later and scarify/mow (only) again in the autumn.

Welcome, Theo!
If you garden in a mild-winter area, like I do, you'll want to scarify tropical grasses--such as Bermudagrass or couse--the beginning of their growing season. April where I am, but earlier in more tropical areas, or later in coastal areas with cooler summers. Cool-season grasses--such as bluegrass or creeping fescue--should be scarified in the fall. Clumping turf grasses--such as tall fescue or perennial ryegrass--probably shouldn't be scarified, unless you are going to re-seed.

Thanks very much for the info.

Regards

Theo.