I remember learning that someone's assassination is the day the person is shot (or otherwise harmed), not necessarily the day they die. So someone can be assassinated months before their death. I can't seem to find any proof of this, though, other than Wikipedia listing the dates of the assassinations I've looked up as the dates of the initial incidents.
Today is the anniversary of William McKinley being shot, although he didn't die for another week. So is today the anniversary of McKinley's assassination?
I don't have a direct link to the AP Stylebook, but I remembered from my days in newspaper journalism that their rule always was that if a prominent person is killed one day, but dies another, they're officially assassinated "on the day they were attacked."
This is borne out via this review of various AP Stylebook rules on a different site. I sent a message off to the AP for clarification and will add that if I hear back in a timely manner.
The AP's rule was the only one I've ever seen specifically addressing this manner, sadly.