In general, categories should be pluralized when they are specifying a type of something (e.g. Spells, Rings, Funguars, etc.). If its an event title or something then it should be singular (e.g. Skirmish, not Skirmishes).
Also bugs me when this is inconsistent, but it can be a huge pain in the ass to fix categorization down the road once many pages are fit into a category.