The Turning at Labor Day "Season of mists and mellow fruitfulness" - well, not quite yet. It is still a little early to have John Keats's autumnal words running through your head. Yet no matter how desperately you cling to summer, it's almost impossible not to feel the changes that begin to gather around Labor Day. On a warm, muggy day, they are not very evident. Only the shifting of the light at evening suggests the changing season. But when the wind is fresh and the temperature falls a little, it is hard not to imagine the season leaping ahead of itself, even as summer lingers. The promptings are obvious in the country - ripening tomatoes, more sweet corn than anyone knows what to do with, piles of cordwood heaped near woodsheds. Here and there, well north of the city, a roadside maple has jumped the gun and is already flaming. But the real ripening everywhere is human. Whatever students may feel about returning to school, it causes their parents to wonder how their offspring got to be so old and to try not to wonder what it says about themselves. For every family with school-age children, Labor Day, not New Year's, is when the new year really commences. The city is still a heat-sink, and it will be a while before the subways lose what still feels like mid-July's steam. Long Island Sound is still warm, and it, too, will be slow to lose its heat. Like Memorial Day or the Fourth of July, Labor Day has nothing to do with the rhythms of nature, nothing to do with the movement of the sun. It just happens that we pause every year about now and look around us and notice the way the small changes add up. It's a reminder that we could do this almost any day of the year: declare a holiday, stand back, and consider the ebb and flow of the world we live in.