In my experience either March or April is fine. But, the best month of the two is March. I have worked as a pest management professional for over 25 years and I have spent a good 15 years working on ways to control different weeds and grasses. I suggest making any applications to control dandelions at the beginning of March. At this time the dandelions should be actively growing and young, they are easiest to kill at this point. I suggest you purchase your pre-emergent to control the dandelions from a garden store, Home Depot, or Lowe's. Ask an attendant for their recommendation and follow the label directions. If pre-emergent you use comes in the form of a granule I suggest you water your lawn first. This will help the granules stick to the vegetation and the water will help the pre-emergent to break down and start killing you dandelions. If you have to make another application that will be fine as long as you follow the directions on the label.