Well, yes, but no one is suggesting that making a single large buy on one particular day - any day - and no other investments at all after that, is a sound stra

Well, yes, but no one is suggesting that making a single large buy on one particular day - any day - and no other investments at all after that, is a sound strategy. Considering that the time period you're talking about saw: the Dust Bowl, the rise of Fascism, a global conflict on a scale never before seen, industrial genocide, and the start of the Atomic Age and with it the threat of nuclear war and the annihilation of our species... the fact that a 20 year-old could have made the insane investment choice you've presented, and still managed to break even by the age of 45, is actually quite remarkable. (And I'll just ignore that you've not accounted for dividends.)