Crabgrass preventive is applied prior to germination, broadleaf weed killer needs to be applied to growing leaf surfaces.  The best time to spray is in the fall, when they are moving sugars and starches from the leaves to the roots for winter storage. You will get best movement of herbicides into the roots at this time. The second best time of year for spraying is during or right after the time the first flush of dandelion flowers appear in the spring. However, spraying in the spring increases the risk of damage to other plants, including trees and shrubs, from the herbicide. Spray during mild, but not hot, weather, when there is not a wind that might blow spray onto desirable plants. Avoid spraying in the rooting zone of trees and shrubs, if possible. Make spot applications, if possible, rather than spraying the whole area. Granular herbicides are not likely to give satisfactory control of dandelions, so I do not recommend them. Use a dandelion digger if you can, rather than a spray, digging out a couple inches of the root to prevent growback. Maybe a neighbor kid or child in your family would be happy to dig dandelions for 2 to 5 cents each! Dandelions, crabgrass and other weeds are more likely to be a problem if the grass is not thick, so whatever you can do to thicken the lawn, should help, too.