My friend Ellen called yesterday. ''Let's hear it,'' she demanded. ''Letting Go, 101. Give me the cram course!'' Apparently, Ellen's five-year-old son was loading and unloading his new backpack in readiness for his first day of kindergarten, and Ellen was feeling those ''left in the dust'' blues. ''This is nothing like day care,'' she observed. ''I'm not leaving him now. He's leaving me!'' How could I help? When each of my four children ventured forth on their first day, I felt exactly like Ellen. I marveled how easily they walked away from me, skipped even, when my own knees barely held me steady. My bright kindergartner would look back and wave, then turn and walk away even faster. Let them go? It was much more a matter of not letting myself follow. ''You'll really appreciate the miracle of the camcorder now,'' I said to Ellen. ''You watch your child walk away through the eye of that lens, and you're much too busy recording the moment to make a blubbering fool of yourself. Even if you get the urge to run along behind the school bus, a camcorder is much too bulky to let you do it with grace.'' Ellen laughed. ''But why does it have to hurt so much?'' she asked. I had no answer for that. For me, subsequent autumns don't get much easier. The ache I feel when the smells of chlorine and suntan lotion give way to those of crayons and new shoes reoccurs every year. It's familiar now, and as September settles in, I know the ache will ease. This fall holds a first for me, too. My oldest child will be entering high school. When I mention this to a neighbor, I roll my eyes and shake my head, and the woman never suspects that my bravado is all pretense. Reality doesn't accommodate my nostalgia any more than it does Ellen's, though. Of course, my daughter will go to high school - and I bet this year she won't even let me use the camcorder as a cover. Isn't it natural to hold on tight to our kids? Isn't it unnatural to let go? Yet that's what parents are expected to do. We're even supposed to let go so smoothly that our kids don't even notice! Like when we taught them to ride that two-wheeler: With a steadying push, we unclenched our hand from the seat and watched them weave uncertainly but most assuredly away from us. We stand there watching that first shaky baby step, that first scramble up the school bus steps, that first wobbly ride down the sidewalk. We stand there, not daring to breathe, certain the slightest movement on our part will upset the delicate balance so proudly displayed before us. Before I became a mother, I remember thinking that parenting requires great leaps of faith. The discomforting twist I've discovered is that parents supply only the faith, with the attendant prayers; all the leaping is done by the children. And that first day of school, no matter how old my kids are, is one more leap on their perilous, jubilant journey away from me. My annual autumn ache is actually a helpful prod - Look! before it's too late.... That's the answer to Ellen's question, then - why letting go hurts. Despite the dead days of August when our kids' boredom and bickering makes ''Come, September!'' an absolute imperative, the first day of school is not only one of beginnings, but of endings as well. The accompanying ache forces our eyes open so we see suddenly how old our children are getting. Somewhere along the way, when we were not looking, they learned that the blankie can be left behind; that the dirty clothes actually go in the hamper; that poetry doesn't have to rhyme; and that Tchaikovsky is worth listening to, at least some of the time. Nothing earth-shattering, but each wondrous in its own way. That's why I make the first day of school, each precious one, my day for looking. Not back to the past with longing nor ahead to the future with apprehension, but clearly at the present: out to the street where my children cavort at the curb, or chat nervously with friends, or look bored and much too cool for anything as childish as waiting at the bus stop on the first day of school. I watch them and try to absorb it all till I'm saturated. In a moment, the yellow bus will round the curve and coast to a stop. Then my children will be on their way, and the street will be silent and empty once more. Except for my gaze that will linger there. And my prayer.