A friend recently realized that the homecoming dance her teenage daughter attended in the fall was her last, because Alex is a senior. It was one of many such events in Alex's busy life, and so the finality of it went unnoticed until weeks later. My friend was sad not only for the realization, but for not having recognized it at the time.

At first I didn’t get it, why this might be painful for her, especially when I asked if she’d have done anything differently had she known. She said, "Well, probably not. But it's still sad to know it was the last."

And I guess it is sad, not simply that things end, but that we don't always recognize that end until later, when it's too late to go back. We’re left with only the memory, and the sense that something has ended without our consent.

Firsts, on the other hand, are usually anticipated and often celebrated, especially for parents: The first time our baby smiles at us. The first time she rolls over, sits up, takes a step. Then there's the first bicycle, the first haircut, the first lunch box, the first bus ride. There are first dates, first kisses, first heartbreaks.

When we think of firsts, we don't always think of lasts, but they're all connected. When a baby learns to walk, he no longer crawls. While on some level I understand that “Jimmy Neutron” means no more “Teletubbies,” it needs to be unconscious; I simply wouldn’t be able to function if the knowledge of those lasts - the last time I kiss my husband, or see my children's faces, or hug my mother - was perpetually on my conscious horizon.

My friend David recently died, and in the days following I kept going back to our last conversation. I was angry with myself for the fact that it was so normal, as if I had assumed I'd be speaking with him again the next day. Which, of course, I had. I couldn’t get past it to let myself grieve.

Then I wondered, what would I have said differently had I known it was my last chance? Maybe I would have told him what a good friend he was, how special he was to us, how glad I was that he was a part of our lives. Maybe I would have apologized for not being there sometimes, for being too busy to call him back sometimes, for being too self-involved to pay attention sometimes.

But I don't think so. I think the enormity of knowing what was to come would have been too overwhelming to allow me to say anything more than what I did: "Okay, gotta go; call if you need anything." The rest, I’m hoping, was said through the years, over dinners and plays and hospital visits and trivia games.

A last is like an alarm on the clock of our life; it tells us it's time to move on, and if we can't make that move ourselves, then nature and God do it for us. The lasts dictate that babies learn to crawl even if we prefer they stay infants; that children get on the bus though we might prefer they stay toddlers; that teenagers leave us and go to college though we might prefer they stay children.

They dictate that people we love should sometimes find peace by dying, though we might prefer that they live.

It is almost the last day of the old year, and the first day of the new. We cannot have one without the other. And even while we mourn the passing of a phase of our lives, we must at the same time celebrate the new beginnings it creates. Alex is going to leave home and become an amazing woman. David is never going to feel pain again, ever.

And I think I might just be getting it, at last.