It's Christmas Eve, the house dark, our shadowy parents hunched at either end of the candlelit table. I'm 9. I'm leading my two younger sisters as we circle the table. Every time I pass a parental silhouette, that parent thunks something onto the plate I'm carrying. I yank the plate close to my face. I squint at it, sniff it. Behind me, my sisters do the same. Is it another Christmas cookie? More marzipan? A handful of cashews, a tangerine? Or is it . . . the onion? The onion on your plate is the vegetable equivalent of a lump of coal in your stocking. Each time I dodge it, I say something clever like, "Whew! I got marzipan!"

Our German immigrant grandparents brought a Christmas plate tradition from the fatherland. On Christmas Eve, they'd set a plate out for each child, already filled. By the time we came along in the '60s, the tradition had evolved to include a plate-filling parade around the table. And the onion. Though sometimes it was a pickle. Occasionally, a radish.

Thunk. Thunk. Thunk. Behind me, my youngest sister, Erin, stops. It's the onion, she can smell it. Erin is 3. She doesn't yet realize one must accept one's fate. She hands it back. Watching this, Ingrid and I understand: One of us will have to take the onion hit for our baby sister. Two rounds later, thunk, it hits Ingrid's plate. Being 7 and hip to the ways of the random universe, Ingrid shrieks. She feigns major trauma. Because, after all, it isn't an onion, one of many. It's the onion, the one and only Christmas Onion, and its arrival on your plate is the gods' way of intoning, "Little girl, we consider you worthy to anoint as the special loser of this special night."

A year later, our parents will split up. But the Christmas plate tradition will continue. In fact, it will double. One week we parade past our mother, the next our father. Back and forth. Parents, and, later, men, will come and go. But we sisters will weld ourselves into a mobile, self-contained unit.

As adults, our favorite collective holiday memory, the one that defines Christmas for us, will be those plates and the parade and the onion, fun and a little mysterious.

I call our mother and ask about it. "Why did you add the parade and the onion?"

"What onion?" she asks. She can't remember but thinks it sounds like our father's idea.

He can't even remember the parade. "Oh yeah," he says when I helpfully remind him it was the highlight of every Christmas season. "I think we added the parade to fill some time."

And the onion? According to our father, it probably just seemed like a good joke.

It's Christmas Eve. I sit with my sisters around a dining table while their children parade past us with their Christmas plates. By now jester hats have been added to the kids' heads, and music, and this: a disgusting vegetable for every child -- (a) because they all want one to shriek over, and (b) because, after all, the universe really isn't so random: Eventually we all wind up with an onion on our plate. Watching them, I realize the meaning of the Christmas Onion is the meaning we give it: a gentle warning about life, Achtung, Kinder! Life may hand you an onion, but you still have marzipan! In the case of our parents' divorce, we sisters had one another.

Kristin Henderson is a frequent contributor to the Magazine.