Full credit goes to this Flyertalk post…

Oneworld member Air Berlin is having a 1-day sale on Wednesday December 19th at airberlin.de and you can get roundtrip tickets from New York to Berlin and Dusseldorf for just under $400, taxes included. It seems to only be available at the German version of the Air Berlin site (though on the upper right hand corner you can switch to English). Flights are available in January and February.

And while I’m writing this Tuesday night, it’s already Wednesday in Germany so the sale is currently live.