Sometimes cats pick up on emotions and super charged feelings. Just something to think about. I don't really believe in actual ghosts as in spirits still on earth kind of thing. But I do believe somewhat that people leave behind something sometimes when they go. Some kind of resonance or energy. And it may or may not be stronger on or around the day they died. He could be sensing this. But, as a precautionary measure, since he is quite old, I suggest you visit a vet because his old age may be catching up with him. Good luck...give him hugs!!