Late-spring to early-summer is the critical period for control of nearly all major broadleaf weeds, including dandelion, clover, spurge, plantain and dozens of others. Apply a selective, water-based herbicide that kills all major types of broadleaf weeds down to the root, yet won't harm lawn grasses. For weeds that resist treatment, follow up in the fall. Fall is actually the most effective time to kill many weeds, because they are actively sending nutrients from leaves to roots for over-winter storage. Weed killer applied to the leaves is very efficiently transferred to the roots. In addition, spot-treating weeds that pop-up during the summer with a ready-to-use weed killer will remove those unwanted intruders. 

Minimal: One treatment of the entire lawn during late spring when broadleaf weeds are just emerging (but have not yet set seed) is fine for exceptionally lush, nearly weed-free lawns.