It seems that we may have some questionable synonyms. For example:
There are two different concepts. So far I believe we've only had QA on one of them:
There's a festival which goes by several names and several spellings which is celebrated in several ways about the same time of year in several places around the world. This tag should be singular, since they are manifestations of the same thing. But having more than one well-known name for the event in one tag is a good idea:
Carnaval/Mardi Gras, when written as permitted by tags would become
There are many small seasonal, sometimes travelling events, which are known in English as fairs, funfairs, carnivals, etc. These should have a plural tag since they are separate things. I think we could again go with a multiple name to avoid confusion:
Carnivals and fairs, when written in Stack Exchange tag syntax would become
I've noticed questions for 1. but not for 2. yet though it's possible I've missed some.
Variations and discussion on this theme is welcome of course.