As you may know, Robin’s friends and family had a memorial for him on September 27, 2014. His close friend Billy Crystal opened the memorial in tears. I read Robin’s daughter Zelda organized the event which was only for his friends and family. There was not a public memorial for Robin. After all, he was a very private person and his family respects his final wishes. Sorry, I don’t have the link to the article but you may be able to find it by typing Robin Williams memorial into your browser.
Memorial For Robin Williams