Mehefin comes from Proto-Celtic medyo-samīno- (“middle of summer”), like Irish Meitheamh.
Does mis mean month?
Is this the genitive case meaning the month of?
So mis Mai can also the month of May?
You could say that, yes. Although Welsh doesn't have separate genitive forms of nouns, "the month of May" is a valid translation.
how unimaginative a phrase!