1975 seems to be the universal date that i've been finding while looking, but the stories contradict themselves. One says it was when he and Linda were eating lamb and were looking out the window at all the little lambs running around which turned them off. The other says it was when Paul was on a fishing trip and felt sorry for the fish he was reeling in. Whatever. Maybe i'll look into it further later on, but I need to go flip the bacon right now.