TOKYO — Christmas is coming, so it’s time to reserve fried chicken and cake in Japan.

Consumption of those two foods has so firmly come to mark the informal holiday here — Dec. 25 isn’t an official day off — that few Japanese are aware other countries prefer somewhat different Christmas cuisine.

Where Christmas means KFC

“I had no idea,” said 22-year-old university student Ryuya Morimoto, informed that chicken and cake aren’t American yuletide staples. “When I was a little kid, we would eat chicken as a family,” he said, “and then have Christmas cake, with a little chocolate piece on the top that said, ‘Merry Christmas.’ ”

Families like his make this the busiest time of the year for Kentucky Fried Chicken outlets in Japan.

KFC YUM, -0.90% lets customers reserve fried chicken as early as October’s end. Those who try buying buckets on Christmas Eve risk having to stand in line up to six hours.

An expanded version of this report appears at WSJ.com.