We bought Scott's Turf Builder when Walmart was clearing it out for $4.00 and we had a $4.00 coupon.
We apply it in the spring and fall, when it is raining out (we don't water the lawn -- too expensive).

This is great stuff. We aren't gardeners but we followed the directions and our lawn looks like a lush green carpet. The dandelions have nowhere to grow on our lawn because it all lush green grass.

If you can find a deal on Scott's Turf Builder, I highly recommend this.