There is no official announcement from Google regarding algo update on April 17th & 18th. But many webmasters faced traffic dance for their website in GA. Was their any update? If Yes !! What is that, Fred, Panda or mobile first index.
I have analyzed all my client website. I can not conclude what the update was.
If you said its Panda, We are have 3 test website with same content. After April 16th the traffic was improved.
So it might be mobile first index or Google dance