It seems like all my life I have heard that voters do not pay attention to the presidential election until after Labor Day. I did not hear that once this summer and it seems like (from the polls) they had all made up their minds long before Labor Day.I've been hearing this a bunch this year, and I think it somewhat conflates a couple of things.
There was an old tradition that the campaign didn't start until Labor Day. I'm not sure how true that ever was (as long as I can remember, it was mentioned only as an old tradition now violated), but remember that there's a history to these things. In the very old days, candidates didn't do much in-person barnstorming. Then, as that norm broke down, recall that until the nomination process reformed most parties didn't know who the nominee would be until the convention. So while conventions were somewhat earlier than they are now, the winner would have to begin organizing a campaign, often from scratch for those who didn't run in primaries, after the convention was over. And given that August is never a great month for getting people to pay attention to politics, it makes sense that the Labor Day tradition -- or at least the belief that there was a Labor Day tradition -- got started.
Now, it's probably still true that a lot of people don't pay attention until something cues them that it's time. That might be the conventions; it might be TV news gradually shifting to make the campaigns a focus of their coverage; it might be the debates. But thanks to partisanship a lot of people either make up their mind before they start paying attention, or think of themselves as undecided but whatever it is that cues them to start paying attention also activates their partisan preferences.
So the upshot of all this is that both things are true: most people don't pay a lot of attention before Labor Day or so, but there are also relatively few undecided voters by the time we get to Labor Day.