Dethatching is an important component of lawn maintenance. You should dethatch if thatch is more than ½" thick. Use an iron rake or a thatch rake (also known as a "scrake") to cut through and rake off thatch. This will also scarify the surface. For large lawns, you may want to consider renting a walk-behind dethatching machine.
Contrary to common homeowner practice, you should never dethatch in mid- to late spring or during your lawn's active growing season. Because it exposes the soil, dethatching can give weeds a chance to take over your yard. Instead, dethatch your lawn in the fall after growing season.