Merry Chrismukkah! Or is that happy Chrismukkah?

Whatever it is, it's back tonight on "The O.C." at 7 p.m. on Ch. 13. It's a don't-you-dare-miss-it episode for fans of the show . . . and one that will make fans out of first-time viewers.

Chrismukkuh is, of course, a combination of Christmas and Hanukkah, created by Seth Cohen (Adam Brody), the son of a Jewish father, Sandy (Peter Gallagher), and a Christian mother, Kirsten (Kelly Rowan).

Or, in reality, the invention of "O.C." creator Josh Schwartz, who "grew up as a lot of young Jewish boys do — envious of all the Christmas holidays that went on.

"What Jewish boy or girl . . . doesn't feel a little jealous? They get all the good songs. They get the tree. They get all the characters, Frosty and Rudolph. You know, we got dreidels. Not the same thing.

"We wanted to create a holiday where both religions and both holidays are celebrated."

The word "Chrismukkuh" was pitched by someone (Schwartz can't remember who) in "The O.C." writers' room.

"I wanted to come up with a holiday sort of in the vein of Festivus from 'Seinfeld,' " Schwartz said. "So we started bandying about interfaith holiday names (like) Hanumas. And then someone in the writers' room said Chrismukkuh, and that was it."

What makes Chrismukkuh so utterly charming is Seth's utter belief in it. He's sort of a latter-day Linus in the pumpkin patch, certain that the Great Pumpkin will appear.

"Seth is very big that this is the year that Chrismukkuh is going to sweep the nation. . . . And he's also fully expecting the backlash," Schwartz said.

This year also marks the debut of . . . the Yarmuclaus. It's the Santa/Jewish hat (see accompanying photo) that leaves Seth almost speechless with delight. "Holy Moses, it's beautiful," he says. (You can buy them — along with Chrismukkah wrapping paper and cards and so on —at theocinsider.com.)

There's more to the episode than just Chrismukkah, however. There are some big soapie events, like ex-spouses Jimmy (Tate Donovan) and Julie (Melinda Clarke) hooking up while Julie's current husband, Caleb (Alan Dale) — who is Kirsten's father and Seth's grandfather — has to reveal to everyone that he has an illegitimate teenager daughter, Lindsay (Shannon Lucio), by a city councilwoman. This would be the same Lindsay that Ryan (Benjamin McKenzie) is dating — so she's his foster mother's half sister.

But just when things seem to be darkest, there's a Chrismukkah miracle.