Steps to replicate the issue** (include links if applicable):
Use {{CURRENTMONTHNAMEGEN}}
What happens?**:
Displays January (or February/March/April/May/June/July/August/September/October/November/December) as appropriate. In English, these are the common case forms of the month names.
What should have happened instead?**:
The correct English genitive case of January is January's. Similarly, February is inflected to February's, March to March's, April to April's, etc.
Other information** (browser name/version, screenshots, etc.):
The incorrect system messages are [[MediaWiki:January-gen]], [[MediaWiki:February-gen]], [[MediaWiki:March-gen]], etc.
Yes, this is very pedantic, but I am right about the grammar, and it's annoying.