The term ‘Christmas Cake’, although becoming somewhat outdated, is still used in Japan to refer to a single unmarried woman who is older than 25. It is said that no one wants to buy a Christmas cake after the 25th (no matter how delicious it looks), and in turn, no one wants to marry a woman after she’s 25 (no matter how beautiful she is). This term is not used for men. In sort, being unmarried after 25 means you are “past your due date”.
A “Christmas Cake” woman is commonly used as a trope for characters in anime and manga as well.