KFC’s massive advertising campaigns have embedded “KFC = Christmas” into the minds of many Japanese. Christmas in Japan is purely commerical with no religious strings attached. So, just about everything the Japanese know about Christmas has been fed to them through television , radio , and myriad magazines, including commercials for chicken.
BTW - I live in Japan .
I thought KFC got started in the Christmas biz because so many Japanese started asking for the bird they saw served in all the Christmas movies that came from America... as they thought it a giant chicken (being that turkey doesn’t exist in Asia, unless imported).
So KFC, noting that Asian kitchens don’t have ovens, sensed that this would be a good business move and started to offer whole-chicken take-home dinners for Christmas. So they now heavily advertise it every year.
BTW, they do the same in Korea as well.