Can't believe they call it that.

Vagitaria..?

Pen night works, for those who know a little bit about swans, but it'll never catch on.

Fen night, 'Dutch night' and Zuider Zee Revel all work well, what with the obvious connotations about dykes. But somehow, I think that someone will be terribly offended.

We need a classicist to offer inspiration, here.