I am in a couple Facebook groups. When people get their Interview letter they post it on Facebook, on the group page.
If you are on Facebook search for the group... Nigeria/America Immigrant visa process for CR1/IR1
is a very informative group. People share their progress and ask questions.
I hope it helps you.
The last Interview letters were given towards the end of May. I am hoping that by next week the next batch of interview letters will be given out.