Anything that happens at the same time, but which has no causation is coincidental. It's unfortunate. Lewis and Huxley might be over-looked on the anniversary of their deaths as we are more aware of Kennedy. His death was more shocking and newsworthy. However, the happenstance of their deaths on the same day is trivial.
While I believe it is possible that there is a coincidence in the deaths of JFK, C.S. Lewis, and Aldous Huxley, I would not be able to explain what it was. I believe God works in mysterious ways and His ultimate plan is so intricate and complicated that we would never be able to even begin to comprehend it. There may be a coincidence between all three deaths because everything happens for a reason.
I don't think there is any coincidence in these deaths, the fact that these people were famous and died on the same day is not that much special. Counting all the population throughout time, there has to be days when people or "famous" people die in bigger quantities than others, of course there will be dates where that will happen.
I don't think that there is any reason to correlate their deaths because I believe it was a coincidence, much like a lot of things people tend to tie together and say "This happened because of this." Nope, this is just the way works and is how it always will.