Currently, the tag christmas cake is used for images of "mature women with reference to either their age [usually 25+] or the crippling, desperate loneliness and depression which consumes their existence," in reference to Japanese slang (クリスマスケーキ).
However, doing some light research online indicates that, in Japan itself, the term has largely dissipated and become uncommon as a result of shifting trends, mainly the trend since the 90s of women increasingly remaining unwed, with the average age at first marriage circa 2009 being 28 for women, and 30 for men. Terms such as the wiki's mention of urenokori (売れ残り, うれのこり) have since become more common, though in other circles you might see Chinese terms such as sheng nu or 3S.
Additionally, having this tag occupied by outdated slang means that actual, genuine Christmas cake can't be tagged in the same fashion as strawberry shortcake, swiss rolls or birthday cakes. That's how the equivalent `christmas cake` tag on Pixiv is used.
With all this in mind, it would probably be worthwhile discussing what to do with the tag.