Well, a week or two, actually. The motorcade route was published the week before, and it just happened to pass by where he was working.
Conspiracy theorists could only thrive in a media-soaked world such as ours, where large, complicated schemes happen nightly on movie and TV screens, and those watching them mistake that stuff for real life.
In real life, nothing ever comes off the way it is planned, and the more people you involve, the more things go wrong. Not to mention that, no real conspiracy would involve LHO, who "did not play well with others".
I still like the Donohue theory, and nobody has shown me a better one for explaining all the physical facts, and even a number of the psychological ones, so far. And it involves only one (very small and understandable in the circumstances) conspiracy, after the fact.
Oswald murdered Kennedy.
My personal favorite theory is that he was shot from in front and his head flew back and his brains blew onto the back of the car, where Jackie climbs out and picks them up in her hand.
Oh wait... that’s not a theory, there was film of that...