Our friends at Wikipedia tell us that the PayDay was introduced by Frank Martoccio in 1932. Martoccio served at the head of the Hollywood Candy Company in Centralia, Illinois. Hollywood was sold to Consolidated Foods in 1967, which later became Sara Lee. After the plant was destroyed by fire in 1980, the PayDay was produced by L.S. Heath and Sons Company until a new plant could be built. In 1988 the company was purchased by the Leaf Candy Company, which became part of Hershey in 1996.
In addition to this info, there was a big uproar in 1996 when the main plant in Centralia was going to be closed. There was a large and far-reaching campaign to save the plant. This is, of course, the same plant that is featured in Michael Moore's 1997 film The Big One. Many of you may remember the scene where the workers put the PayDay bars into the coffin in protest.
The plant was ultimately closed.
None of this explains the actual origins of the candy bar itself - why did Frank Martoccio make it? I guess that's what happens to history when the company is steeped in drama - it drowns out the rest of the history.