One of the world's quirkier holiday traditions is now upon us. Every December 24, thousands and thousands of people in Japan leave the house and head to Kentucky Fried Chicken for the traditional Christmas Eve meal:

Yes, that Kentucky Fried Chicken. KFC is so popular in Japan on Christmas Eve that there are often lines stretching down the block outside stores. Some people reserve their KFC "Christmas Party Barrels" up to two months in advance so as not to miss out. A party barrel goes for ¥4,090, or $38, and includes a Christmas cake:

Having grown up in Tokyo, I can confirm that this is a real phenomenon. It's not uncommon to see celebrity-filled commercials reminding people to pre-order their holiday chicken buckets. KFC officials have said they sell more chicken in Japan on December 23, 24, and 25 than they do in half a regular month. Stores are so packed that back-office executives reportedly have to help out behind the counter.

The origins of "Kentucky for Christmas"

What makes this so unexpected is that Christmas isn't even an official holiday in Japan. Rather, it's the result of a savvy marketing operation that's been going on for decades.

It all dates back to a savvy marketing campaign from the 1970s

According to KFC's Japanese website, the tradition started in the early 1970s — shortly after the fast-food chain had opened a few initial stores in Nagoya, Kobe, and Tokyo. As the story goes, an expat came into a store in Aoyama on Christmas, complained that turkey was impossible to find in Japan, but was happy to settle for fried chicken instead.

A store employee suggested to his bosses that there was potential here. And in 1974, KFC launched its "Kurisumasu ni wa kentakkii!" (Kentucky for Christmas!) campaign. As so many Western multinational firms do, KFC was always looking for subtle ways to adapt to foreign markets.

At the same time, Japan was adapting Christmas for its own purposes. Less than 1 percent of Japan's population is Christian, so very few people celebrate the holiday for religious reasons. But starting around the 1960s, the country — particularly its department stores — began fully embracing the more festive, commercial aspects of Christmas that are prevalent in the West.

Every winter, Japanese stores now break out the plastic trees, snowmen, and Santas. Cities festoon their streets with lights. Holiday jingles become ubiquitous. Gift-giving is common. And there are few unique twists — Christmas Eve has become a popular romantic occasion in Japan, akin to Valentine's Day, and couples often go out for dinner dates that night.

KFC found a way to fit into this all (it presumably helps that Colonel Sanders makes an excellent Santa Claus). Back in 2010, the Financial Times reported that other fast-food chains in Japan — like Mos Burger — were trying to get in on the action by offering their own Christmas chicken dinners. But it hasn't quite worked. Christmas means KFC. And KFC means Christmas.

Further reading: Here's a great piece on how Christmas became huge in China.