A cynical, outdated Japanese expression for unmarried women older than 24. As little as 20 years ago, it was expected of a Japanese woman to marry before that age. If she didn't, something was wrong with her, and she probably would end up an old maid - like a cake made specially for Christmas, for which there is little demand after December 24th.
Nowadays, the average age of marriage among Japanese women has risen to 26 years (as of 1995), and marriage at an early age is not anymore considered quite as important.