Hi Diane - it can take anywhere from 9-14 days for the official letter to arrive so you should be getting yours soon. In the meantime start writing those letters - number them in the bottom corner so once you do mail them out your SR will know what order to open them in. Hang in there - it shouldn't be much longer!
Others that have had someone arrive on April 1st and April 2nd are just now starting to receive their form letters. As B'wNukeMom said, you should have yours soon.