Early spring. Purchase your crab-grass preventer in advance--any
brand will do as long as you follow the manufacturer's directions.
Carefully watch your neighborhood for the first dandelions in bloom.
When you see that, apply the crab-grass treatment that same week.
Mark your calendar (usually 90 days) after the first application.
Apply the second application at that time. Do not disturb the ground
with raking, aerating, etc as that will break the barrier. You won't
need a third treatment. In the fall, overseed your lawn. Crab grass
has a tough time growing in a thick lawn. My lawn no longer needs
crab grass preventer, but it took me 3 years to get there. Crab grass
seed can remain dormant for over 20 years. Sunny bare spots are
favorite areas for crab grass growth.