https://en.wikipedia.org/wiki/Christmas_cake#Japanese_metaphor

In Japan, women had traditionally been expected to marry at a young age, and those who were unmarried after the age of 25 were metaphorically referred to as (unsold) Christmas cakes (クリスマスケーキ) in reference to items which are still unsold after the 25th. The term first became popular during the 1980s but has since become less common because Japanese women today can remain unmarried with somewhat less stigmatization. An equivalent term does, however, still exist that hearkens to the “unsold” nature of unmarried women, urenokori (売れ残り, “unsold goods”).