Finally some response to my question!

Back in MY day, the expression "back in my day" was commonly used among the older generation, when they were comparing their younger days with the present. And then my mother had a laugh not too long ago when she heard me say "back in the old days" in reference to my own childhood. I guess the idea of the 1960's being "the old days" struck her as funny. Or the thought that such a young whippersnapper as me knew anything about "the old days" ...

Here's my own suspicion about "back in the day". It sounds like it could well be an old expression from some specific region of the States, and that it stayed comfortably and happily in that region until some local boy went off to L.A. and made good as a script-writer, and put the expression in the mouth of some character in a TV serial or movie popular with the younger set (jeesh, I feel like an old fuddy-duddy!). But that's just a theory. Mack: where is/was your mother from? (She would be about the same age as my mother, by the way).